fix(pinning): use runfiles lookup

[alexeagle]

This allows pinning to work with --noenable_runfiles, for example on a typical Windows setup.

Fixes #899

[alexeagle]

Hm, this won't work with Bazel 4.0.0 - do you want a workaround so we can construct an rlocationpath the old way?

[shs96c]

We're still trying to support Bazel 4, so a fix would be appreciated. If it's not possible, we can discuss whether we want to bump the minimum version to 5.x

[alexeagle]

Actually Bazel 5.4.0 is what introduced $(rlocationpath). I changed to $(execpath) and trim off the external/ prefix in the shell script. My understanding is we know the file in this case is always a source file that we wrote into an external repo, so it happens that this will work.

[shs96c]

LGTM. Thank you. I find it interesting to note that I can tell you're using a linux machine for development because of the change to the lock file