Cache rust artifacts only when cargo files are detected

This way we don't turn the `target` directory into a reserved directory if people are not using rust.

Author: calavera

run-build-functions.sh

@@ -685,9 +685,13 @@ cache_artifacts() {
   cache_home_directory ".boot" "boot dependencies"
   cache_home_directory ".composer" "composer dependencies"
   cache_home_directory ".homebrew-cache", "homebrew cache"
-  cache_home_directory ".rustup" "rust rustup cache"
-  cache_home_directory ".cargo/registry" "rust cargo registry cache"
-  cache_home_directory ".cargo/bin" "rust cargo bin cache"
+  
+  if if [ -f Cargo.toml ] || [ -f Cargo.lock ]
+  then
+    cache_home_directory ".rustup" "rust rustup cache"
+    cache_home_directory ".cargo/registry" "rust cargo registry cache"
+    cache_home_directory ".cargo/bin" "rust cargo bin cache"
+  fi
 
   # Don't follow the Go import path or we'll store
   # the origin repo twice.