Switch stamping detection to ctx.configuration.stamp_binaries()

[justinhorvitz]

Currently, rules_rust uses a config_setting that matches --stamp=1 to detect whether the --stamp option is enabled in the current configuration. This PR removes the config_setting in favor of calling ctx.configuration.stamp_binaries(), which is restricted, but available to rules rust because it is on the restricted starlark API allowlist: https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/packages/BuiltinRestriction.java;l=73;drc=135a03ba5dbad5a1a26ef3bde9815ea244a093d7.

The motivation for making this change is to improve Google-internal detection of stamped actions for cross-user caching eligibility. The ctx.configuration.stamp_binaries() call is limited to the case of stamp = -1, whereas the config_setting dependency was unconditional. Notably, for rust_library rules with the default attribute of stamp = 0, we are now able to certify that the rule is not affected by the --stamp flag value.

Google-internal bug number: 419546090