Update Issue Linker regex to only include issues from the same repo

[jonalmeida]

We're using a simple regex here for grabbing the linked issue that will find any issue ID, regardless of repo: https://github.com/mozilla-mobile/android-components/blob/86112e3b34c959e9b47f1503d7f7ee3534203938/.github/workflows/issue-linker.yml#L20

We can update this to only link an ID when it's not part of the same repo. The below expression translates to: do not match if the preceding token is a string value.

(?<![A-Za-z\-\_]+)#(\d+)+

Regex evaluator with tests: https://regexr.com/6rg2v

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