try-runtime: Supporting patching a custom dependency into the branch before the command

[joao-paulo-parity]

Similar to paritytech/pipeline-scripts#41

If one wants to patch a custom repository into the branch before running a command, allow that through PATCH_$REPO. e.g. if I want to use patch PR #11375 of repository $REPO into the branch I'd say /cmd queue -v PATCH_$REPO=11375 -c try-runtime $ [...args]. This can be useful for companion PRs because they're not compiling before their dependencies are merged.

We can facilitate this by extracting the patch logic into a separate script in https://github.com/paritytech/pipeline-scripts, so that it can be used for both the bench-bot and try-runtime configurations.