ping/rust: Don't set PR ref for master build

When building the `master` image we would previously mistakingly set the
ref to the PR commit hash.

This surfaced when testing a pull request from a fork. `master` would be
pointed at the forks HEAD commit hash, but not the fork URL. Thus the
build could not find the commit and would fail. See
libp2p/rust-libp2p#3154

Author: mxinden

ping/_compositions/rust-cross-versions.toml

@@ -25,7 +25,7 @@
 
     [groups.build_config.build_args]
       BINARY_NAME = '{{ .BinaryName }}'
-      GIT_REF = '{{ $.Env.GitReference }}'
+      GIT_REF = "master"
   {{ end }}
 
   {{ if $.Env.GitReference }}