Support env for validate_static_library

[dzbarsky]

We need a way to pass a flag corresponding to "is_darwin" to this action to do the equivalent of

rules_cc/cc/private/toolchain/unix_cc_configure.bzl

Lines 430 to 431 in d81c50d

	# Certain weak symbols are otherwise listed with type T in the output of nm on macOS.
	"%{nm_extra_args}": "--no-weak" if darwin else "",

Passing via ENV is the easiest way. (If the implementation is a prebuilt binary instead of a shell script, it can't really be templated in a repo rule)