pw_toolchain_bazel: Create temporary variable

An internal user has written

```
pw_cc_action_config(
  ...
  implies = ["archiver_flags", "linker_param_file"],
)
```

We will:
1) Submit this CL
2) Switch their code to use this variable
3) Submit the action config CL which changes this to labels
4) Inline it in their code.

Bug: 322872628
Test: None
Change-Id: I4ec05c9f8af1e118f957b69ecd0d5e1e377689bd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/189970
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Matt Stark <msta@google.com>