Fix constructing an Action from its action digest in `process_executor` (#17249)
Fix constructing an Action from its action digest in `process_executor` (#17249)

Fix constructing an Action from an action digest, by filtering out environment variables which are (re-)set by execution request construction (which otherwise cause a failure to set a reserved environment variable name).

Fixes `process_executor` when used with an Action digest produced for the sample server added in #17209. Afterwards, can use:
```
./cargo run -p process_executor -- \
  --action-digest=abcd01234.. \
  --action-digest-length=42 \
  --remote-instance-name="" \
  --server=http://127.0.0.1:50051 \
  --cas-server=http://127.0.0.1:50051
```
...to reproduce a failed process execution.

[ci skip-build-wheels]
pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants