[CI] Elect more builds to run on diff depending on simple heuristic

Summary:
This diff makes the build-bench to run when a diff changes a file in
src/bench/ and the secp256k1 builds when a diff changes a file in
src/secp256k1/.

Test Plan:
This can't be tested before landed, but you can check that the similar
D8943 is working as expected.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8997