dev: builds_for_pull_request(): rename res to builds_for_release

This is a complex function, so let's avoid cryptic variable names
which make it harder to understand
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.
dev: builds_for_pull_request(): rename build_tmp to valid_builds

This is a complex function, so let's avoid cryptic variable names
which make it harder to understand.
dev: builds_for_pull_request(): rename id to build

This is a complex function, so let's avoid cryptic variable names
which make it harder to understand.
dev: builds_for_pull_request(): rename p to parent_build

This is a complex function, so let's avoid cryptic variable names
which make it harder to understand.