feat: read arch from environment

Read the runner's arch from the RUNNER_ARCH environment variable, rather
than hard-coding the current default.

Fixes #38

Signed-off-by: Joshua Lock <jlock@vmware.com>

pkg/provenance.go

@@ -162,7 +162,7 @@ func GenerateProvenance(name, digest, ghContext, command, envs string) ([]byte,
 				},
 				// Non user-controllable environment vars needed to reproduce the build.
 				Environment: map[string]interface{}{
-					"arch":               "amd64", // TODO: Does GitHub run actually expose this?
+					"arch":               os.Getenv("RUNNER_ARCH"),
 					"os":                 os.Getenv("ImageOS"),
 					"github_event_name":  gh.EventName,
 					"github_run_number":  gh.RunNumber,