fix(ping/rust): Don't set PR ref for master build (#79)

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

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 }}

ping/rust/Dockerfile

@@ -11,7 +11,7 @@ ARG GIT_TARGET=""
 RUN if [ ! -z "${GIT_TARGET}" ]; then sed -i "s,^git.*,git = \"https://${GIT_TARGET}\"," ./plan/Cargo.toml; fi
 
 ARG GIT_REF=""
-RUN if [ ! -z "${GIT_REF}" ]; then sed -i "s/^rev.*/rev = \"${GIT_REF}\"/" ./plan/Cargo.toml; fi
+RUN if [ ! -z "${GIT_REF}" ]; then sed -i "s/^rev.*/branch = \"${GIT_REF}\"/" ./plan/Cargo.toml; fi
 
 # Build the requested binary: Cargo will update lockfile on changed manifest (i.e. if one of the above `sed`s patched it).
 ARG BINARY_NAME