bazel: Recommend setting --action_env=PATH for local cache

The old configuration makes it impossible to compile Gerrit on Mac with
recent versions of Bazel. The flag --experimental_strict_action_env now
makes Bazel pass PATH=/bin:/usr/bin to subcommands. Building PolyGerrit
requires using node from $PATH. On Macs it is impossible (even for root)
to write to /usr/bin, so node has to live somewhere not in that
hard-coded PATH. I'm not sure when this behavior changed in Bazel; it
seems like it should have broken long ago. But it's marked experimental
so we can't complain too much.

The documentation of this option says:

    If true, Bazel uses an environment with a static value for PATH and does
    not inherit LD_LIBRARY_PATH or TMPDIR. Use --action_env=ENV_VARIABLE if you
    want to inherit specific environment variables from the client, but note
    that doing so can prevent cross-user caching if a shared cache is used.

Passing --action_env=PATH fixes the problem. I think it should be
safe(ish) to pass --action_env=PATH, since the particular setup
described in our docs is not used for cross-user caching. This may not
catch potential cache invalidations due to different versions of tools,
but I don't see how it's any worse than when the binaries in /usr/bin
get updated.

Change-Id: I94543bd4801b83a66ff150baf3fb9e12ded7555f

Documentation/dev-bazel.txt

@@ -390,6 +390,7 @@ and add these lines to your `~/.bazelrc` file:
 build --experimental_local_disk_cache_path=/home/<user>/.gerritcodereview/bazel-cache/cas
 build --experimental_local_disk_cache
 build --experimental_strict_action_env
+build --action_env=PATH
 ----
 
 [NOTE] `experimental_local_disk_cache_path` must be absolute path. Expansion of `~` is