kitty: fixing environment variable substituition in docs/Makefile

The previous substitutution only worked by accident as make requires
environment variables to be enclosed in curly brackets as shown in this
excerpt from the build output:
> PYTHONPATH=YTHONPATH:.. HOME=MPDIR/nowhere sphinx-build -M html "." "_build" -T

The substituition is moved to the existing patch file to simplify the
builder expression.

pkgs/applications/misc/kitty/default.nix

@@ -35,12 +35,6 @@ buildPythonApplication rec {
     })
   ];
 
-  postPatch = ''
-    substituteInPlace docs/Makefile \
-      --replace 'python3 .. +launch :sphinx-build' \
-                'PYTHONPATH=$PYTHONPATH:.. HOME=$TMPDIR/nowhere sphinx-build'
-    '';
-
   buildPhase = ''
     python3 setup.py linux-package
   '';

pkgs/applications/misc/kitty/fix-paths.patch

@@ -9,3 +9,19 @@
      if (!done) {
          done = true;
 
+--- a/docs/Makefile
++++ b/docs/Makefile
+@@ -3,7 +3,7 @@
+# Patching is needed here for the following reasons:
+# * `sphinx-build` in nixpkgs is not a Python file but a wrapper shell script
+# * importing the `constants` package from Kitty has a side effect that it
+#   creates the user configuration directory. This package gets imported
+#   while sphinx scans the code for documentation strings.
+#
+ # You can set these variables from the command line.
+ SPHINXOPTS    = -T $(FAIL_WARN)
+-SPHINXBUILD   = python3 .. +launch :sphinx-build
++SPHINXBUILD   = PYTHONPATH=${PYTHONPATH}:.. HOME=${TMPDIR}/kitty-build-home sphinx-build
+ SPHINXPROJ    = kitty
+ SOURCEDIR     = .
+ BUILDDIR      = _build