Prefetch Mozilla Rust code dependencies

Make cargo --frozen work when it's called somewhere inside the Mozilla
build system.

build.sh

@@ -55,6 +55,7 @@ popd
 
 pushd "$mozilla_release"
 export MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE=none
+cargo fetch
 ./mach build
 gradle publishWithGeckoBinariesReleasePublicationToMavenLocal
 gradle exoplayer2:publishReleasePublicationToMavenLocal