improve selftest invocation

[jku]

Invoking the test suite manually and in the CI selftest is a little clunky:

• in the CI case workflow generates a shell script that calls the sigstore-python-conformance wrapper that calls "sigstore" binary
• in the manual testing case, using Makefile sigstore-python currently gets installed in the same virtual environment as the test suite requirements: this might be something we don't want (imagine those two needing different protobuf-specs versions for a while).

A possible solution might be:

• make Makefile setup a separate "sigstore-env" virtual environment, like CI workflow does
• make sigstore-python-conformance expect the "sigstore-env" virtual environment (and use sigstore binary from $SCRIPTDIR/sigstore-env/bin/sigstore)

This should keep CLI testing about as easy as it is now and would make the workflow simpler since the generated script would not be needed

[jku]

I'll try to do something about this: testing #182 is already painful without improvements