Move common code into CargoProcess::base.
Move common code into CargoProcess::base.

Both invocations do the same thing with the CWD and the manifest path.
(For `cmd` this moves the --manifest-path flag before the -p flag, but
that doesn't matter.)

The patch also renames base as base_command.

nrc/rustc-perf,nrc/rustc-perf,nrc/rustc-perf