validate --workspace_status_command's value

[laszlocsomor]

Description of the problem / feature request:

Bazel should check that --workspace_status_command's value is a path, not a raw shell command string.

Feature requests: what underlying problem are you trying to solve with this feature?

Remove the need for Bazel to run the command through Bash as part of issue #4319.

The documentation already says the flag expects a path, but nothing validates the flag's value. If the flag's value is a path to a binary or script, then, as long as those are valid executables or have valid shebang lines, Bazel can successfully run them even without Bash.

What operating system are you running Bazel on?

Any.

What's the output of bazel info release?

0.12.0

Have you found anything relevant by searching the web?

Culprit of: #5028
Blocker of: #2190

[laszlocsomor]

Closely related: #4802

[laszlocsomor]

I was wrong, --workspace_status_command cannot expect a path lest it would be unusable in bazelrc files if you run the build on multiple platforms. I guess @ulfjack is right in #5002 (comment), and we must support an actual shell command.

[laszlocsomor]

Obsolete, won't fix.