Silence setlocale warnings in Java stub

Since 17cfa01, the java_stub_template.txt file calls into the locale
tool to check for the presence of cetain locales.  These calls correctly
send error messages to stderr.

Unfortunately, *bash* itself is doing locale lookups as soon as it sees
that the LANG or LC_* variables are touched, and this results in
warnings that pollute the output.  In our builds, the output is
polluted with hundreds of warnings like these:

bazel-out/aarch64-opt-exec-CF41B97/bin/external/bfg/lang/java/src/main/java/com/google/devtools/build/bfg/JavaSourceFileParserCli: line 100: warning: setlocale: LC_CTYPE: cannot change locale (): No such file or directory
bazel-out/aarch64-opt-exec-CF41B97/bin/external/bfg/lang/java/src/main/java/com/google/devtools/build/bfg/JavaSourceFileParserCli: line 102: warning: setlocale: LC_CTYPE: cannot change locale (): No such file or directory

There is nothing we can do to avoid these warnings.  Trying to set the
LANG or LC_* variables via --action_env and --host_action_env has no
effect (other than trigger more/different warnings).

The problem stems from https://bugzilla.redhat.com/show_bug.cgi?id=1361965,
which has been fixed in various distros but hasn't made its way to the one
we are currently using.

Given that it's bash that is complaining, we can easily side-step this
issue by patching the template and wrapping the problematic invocations
behind "env".  In this way, bash never gets to see the locale changes.
This change does precisely this.

src/main/java/com/google/devtools/build/lib/bazel/rules/java/java_stub_template.txt

@@ -97,9 +97,9 @@ function is_macos() {
 function available_utf8_locale() {
   # Both C.UTF-8 and en_US.UTF-8 do not cause any language-specific effects
   # when set as LC_CTYPE, but neither is certain to exist on all systems.
-  if [[ $(LC_CTYPE=C.UTF-8 locale charmap 2>/dev/null) == "UTF-8" ]]; then
+  if [[ $(env LC_CTYPE=C.UTF-8 locale charmap 2>/dev/null) == "UTF-8" ]]; then
     echo "C.UTF-8"
-  elif [[ $(LC_CTYPE=en_US.UTF-8 locale charmap 2>/dev/null) == "UTF-8" ]]; then
+  elif [[ $(env LC_CTYPE=en_US.UTF-8 locale charmap 2>/dev/null) == "UTF-8" ]]; then
     echo "en_US.UTF-8"
   fi
 }