roachtest: use 'master' tag for tpc-e docker images

In [1], [2], we moved tpc-e docker images from the Docker Hub
to a publicly accessible Google Container Registry
(under https://us-east1-docker.pkg.dev/v2/crl-ci-images).
Consequently, most recent changes to tpc-e are now tagged
as 'master'. (This happens as part of the docker GH action
upon merging a PR into master.)

[1] cockroachlabs/tpc-e#71
[2] #117691

Epic: none

Release note: None

pkg/cmd/roachtest/tests/tpce.go

@@ -80,7 +80,7 @@ func initTPCESpec(
 }
 
 func (ts *tpceSpec) newCmd(o tpceCmdOptions) *roachtestutil.Command {
-	cmd := roachtestutil.NewCommand(`sudo docker run us-east1-docker.pkg.dev/crl-ci-images/cockroach/tpc-e:latest`)
+	cmd := roachtestutil.NewCommand(`sudo docker run us-east1-docker.pkg.dev/crl-ci-images/cockroach/tpc-e:master`)
 	o.AddCommandOptions(cmd)
 	return cmd
 }