bazel: fix bash completion, halve build time (#44097)

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.

pkgs/development/tools/build-managers/bazel/default.nix

@@ -128,10 +128,10 @@ stdenv.mkDerivation rec {
   buildPhase = ''
     export TMPDIR=/tmp/.bazel-$UID
     ./compile.sh
-    ./output/bazel --output_user_root=$TMPDIR/.bazel build //scripts:bash_completion \
-      --spawn_strategy=standalone \
-      --genrule_strategy=standalone
-    cp bazel-bin/scripts/bazel-complete.bash output/
+    scripts/generate_bash_completion.sh \
+        --bazel=./output/bazel \
+        --output=output/bazel-complete.bash \
+        --prepend=scripts/bazel-complete-template.bash
   '';
 
   # Build the CPP and Java examples to verify that Bazel works.
@@ -149,7 +149,7 @@ stdenv.mkDerivation rec {
     mv output/bazel $out/bin
     wrapProgram "$out/bin/bazel" --set JAVA_HOME "${jdk}"
     mkdir -p $out/share/bash-completion/completions $out/share/zsh/site-functions
-    mv output/bazel-complete.bash $out/share/bash-completion/completions/
+    mv output/bazel-complete.bash $out/share/bash-completion/completions/bazel
     cp scripts/zsh_completion/_bazel $out/share/zsh/site-functions/
   '';