FR: provide environment variables from key-value integration for pull requests

[magwas]

Description of your issue:

Currently environment variables from key-value integrations are not useable in a pull request due to security concerns.
A solution would be to use the key-value integration of the initiator of the pull request.
My use cases:

• The build expects to have a "issuetoken" environment variable to present, containing a github authentication key, in order to incorporate relevant issues in the build documentation, so for each unimplemented feature the documentation contains a link to the relevant issue (or the lack thereof).
• The build expects a "sonarkey" environment variable containing an authentication key to sonar, so the build can upload the static analysis results to sonar, such that new issues arising due to modifications in the pull request are shown in a pull request analysis there.

See more details in #4928 and #4803

[magwas]

An easier solution: please list all other builds triggered by the same commit on the build page. If the owner of the source repo have set up a shippable toolchain, the reviewer can find the non-PR build for the commit, and look for the information there.