Prepare for 0.0.18 release (#205)

Changelog is
* Use current sigstore-protobuf-specs
* Make sure the test suite is installed in a separate environment in
  both Makefile and the action

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

README.md

@@ -47,12 +47,12 @@ client-under-test [CLI protocol](docs/cli_protocol.md).
           # insert your client installation steps here
 
           # Run tests against production Sigstore environment
-          - uses: sigstore/sigstore-conformance@v0.0.17
+          - uses: sigstore/sigstore-conformance@v0.0.18
             with:
               entrypoint: my-conformance-client
 
           # Run tests against staging Sigstore environment
-          - uses: sigstore/sigstore-conformance@v0.0.17
+          - uses: sigstore/sigstore-conformance@v0.0.18
             with:
               entrypoint: my-conformance-client
               environment: staging