[CI] Properly set the alternative gcc/g++ for arm/aarch64
Summary: The version 9 is installed but actually has no symbolic link by default.
Test Plan: With the bot: build-linux-arma and build-linux-aarch64
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D12115