buildBazelPackage: never append to fFetchAttrs.installPhase

In f8ee061247, the fallback installPhase
if fFetchAttrs.installPhase is not provided, became dynamically computed.
Due to operator precedence this had the side effect of appending to
fFetchAttrs.installPhase if it is provided, breaking custom
installPhases altogether.

pkgs/build-support/build-bazel-package/default.nix

@@ -139,7 +139,7 @@ stdenv.mkDerivation (fBuildAttrs // {
       runHook postBuild
     '';
 
-    installPhase = fFetchAttrs.installPhase or ''
+    installPhase = fFetchAttrs.installPhase or (''
       runHook preInstall
 
       # Remove all built in external workspaces, Bazel will recreate them when building
@@ -183,7 +183,7 @@ stdenv.mkDerivation (fBuildAttrs // {
       (cd $bazelOut/ && tar czf $out --sort=name --mtime='@1' --owner=0 --group=0 --numeric-owner external/)
 
       runHook postInstall
-    '';
+    '');
 
     dontFixup = true;
     allowedRequisites = [];