buildRubyGem: fix to support bundler cmds (#104977)

The way in which Nixpks builds Ruby gems means that certain operations
by bundler *will not work*, namely `bundle install --redownload`.

According to the source the _cache/_ directory should have been kept,
however it seems through revisions to the file it has been purged.

Here was the comment from the original commit that introduced
buildRubyGem:
```
  # Note:
  #   We really do need to keep the $out/${ruby.gemPath}/cache.
  #   This is very important in order for many parts of RubyGems/Bundler to not blow up.
  #   See https://github.com/bundler/bundler/issues/3327
```

Why is the _cache_ directory needed?

Bundler and RubyGems uses the cache as a source of truth.
When bundler executes `bundler install --redownload`, any gems it
discovers in the _GEM_PATH_ it assums must have their _.gem_ file
present in the cache (unaware it was installed from Nix).

Rather than downloading the gem from RubyGems the bundler code forcibly
re-installs the gem from the cache directory instead and **fails** if it
does not exist.

I've opened https://github.com/rubygems/rubygems/issues/4088 to see if
this failure should be soft and not so explicit; or fallback to fetching
the gem from scratch.

Without this change the following is the error:
```bash
> [nix-shell:~/code/nix/playground/jruby-bundler-rake]$ bundle install --force
[DEPRECATED] The `--force` option has been renamed to `--redownload`
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.jruby.ext.openssl.SecurityHelper (file:/nix/store/fis6nzrpw9pmcivr84qh5byfgm07qn10-jruby-9.2.13.0/lib/ruby/stdlib/jopenssl.jar) to field java.security.MessageDigest.provider
WARNING: Please consider reporting this to the maintainers of org.jruby.ext.openssl.SecurityHelper
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Fetching gem metadata from https://rubygems.org/.
Using bundler 2.1.4
Installing hello-world 1.2.0
Bundler::GemNotFound: Could not find hello-world-1.2.0.gem for installation
An error occurred while installing hello-world (1.2.0), and Bundler
cannot continue.
Make sure that `gem install hello-world -v '1.2.0' --source
'https://rubygems.org/'` succeeds before bundling.
```

Wth the fix the following no woccurs:
```bash
[nix-shell:~/code/nix/playground/jruby-bundler-rake]$ bundle install --redownload
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.jruby.ext.openssl.SecurityHelper (file:/nix/store/69wjlj4yirp48rv1q03zxgd4xvf0150d-jruby-9.2.13.0/lib/ruby/stdlib/jopenssl.jar) to field java.security.MessageDigest.provider
WARNING: Please consider reporting this to the maintainers of org.jruby.ext.openssl.SecurityHelper
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Fetching gem metadata from https://rubygems.org/.
Using bundler 2.1.4
Installing hello-world 1.2.0
Bundle complete! 1 Gemfile dependency, 2 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
```

```
[nix-shell:~/code/nix/playground/jruby-bundler-rake]$ ls -l /nix/store/cwl9n5073hqgpfhnw4wic13nrrgg9dn8-gem-env/lib/jruby/gems/2.5.0/cache/
total 8
lrwxrwxrwx 1 fmzakari primarygroup 102 Dec 31  1969 bundler-2.1.4.gem -> /nix/store/ifc8a0gsfkrhkv953rd4rz8bcspahi8y-bundler-2.1.4/lib/jruby/gems/2.5.0/cache/bundler-2.1.4.gem
lrwxrwxrwx 1 fmzakari primarygroup 110 Dec 31  1969 hello-world-1.2.0.gem -> /nix/store/xi9ln6n1mz2is5ppykjxqhhkpjq9zm6i-hello-world-1.2.0/lib/jruby/gems/2.5.0/cache/hello-world-1.2.0.gem
```

I have a minimal project that demonstrates this issue at https://github.com/fzakaria/jruby-bundler-nix-failure

pkgs/development/ruby-modules/gem/default.nix

@@ -49,6 +49,11 @@ lib.makeOverridable (
 , propagatedUserEnvPkgs ? []
 , buildFlags ? []
 , passthru ? {}
+# bundler expects gems to be stored in the cache directory for certain actions
+# such as `bundler install --redownload`.
+# At the cost of increasing the store size, you can keep the gems to have closer
+# alignment with what Bundler expects.
+, keepGemCache ? false
 , ...} @ attrs:
 
 let
@@ -208,7 +213,7 @@ stdenv.mkDerivation ((builtins.removeAttrs attrs ["source"]) // {
     pushd $out/${ruby.gemPath}
     rm -fv doc/*/*/created.rid || true
     rm -fv {gems/*/ext/*,extensions/*/*/*}/{Makefile,mkmf.log,gem_make.out} || true
-    rm -fvr cache
+    ${if keepGemCache then "" else "rm -fvr cache"}
     popd
 
     # write out metadata and binstubs