This finally fixes the build to avoid having to completely rebuild
bazel from source a second time just to generate the bash completion
script!
It also makes completion actually _work_ for bash users by
correcting the name of the installed script.
Bazel either reuses the `PATH` from the client, or sets a hardcoded
one. The former mode in problematic for build hermeticity. But the
latter is crippled on NixOS, because the hardcoded value is
`/bin:/usr/bin`. So we set the hardcoded value to match what
`customBash` provides. This has the effect of aligning the
environments for `ctx.actions.run` and `ctx.actions.run_shell`, which
were previously distinct (bug).
Bazel is a build tool, much like Make and many others. Like Make, it
should be agnostic to the compiler toolchains the user brings into
scope. Bazel has special rules that encode domain specific knowledge
for how to compile a C++ program, or indeed a Java program and a few
others. But that's not to say that at runtime Bazel should assume
a specific C++ compiler or Java compiler anymore than Make does.
The main impact of this change is that packages that build with Bazel
will have to list the compilers they want in their `buildInputs` or
similar, rather than relying on the `bazel` package pulling them in
transitively.
* Skip verifying checksums for already fetched packages.
Needed for two-staged building in Nix:
1. Build a fixed derivation with `bazel fetch` (filtered out of non-reproducable bits).
2. Build an actual derivation which uses fetched dependencies (skipping
checksums needed here because they depend on the build directory).
* Don't clean environment variables for children processes.
Needed for Nix compiler wrappers.
