dev: builds_for_pull_request(): rename id to pr_id
dev: builds_for_pull_request(): rename id to pr_id

This is a complex function, so let's avoid cryptic variable names
which make it harder to understand.  Also id is reused for different
things later in the function.

SUSE-Cloud/crowbar,vwallfahrer/crowbar,crowbar/crowbar,rhafer/crowbar,nkrinner/crowbar,mmnelemane/crowbar,aplanas/crowbar,rhafer/crowbar,MaximilianMeister/crowbar,MaximilianMeister/crowbar,mmnelemane/crowbar,dirkmueller/crowbar,rhafer/crowbar,aplanas/crowbar,tserong/crowbar,nkrinner/crowbar,aspiers/crowbar,crowbar/crowbar,aspiers/crowbar,vwallfahrer/crowbar,SUSE-Cloud/crowbar,rsalevsky/crowbar,toabctl/crowbar,dirkmueller/crowbar,toabctl/crowbar,rsalevsky/crowbar,tserong/crowbar