[CI] Allow for setting environment variables from the configuration file

Summary:
For now this is used by the ARM and AArch64 builds only.

Depends on D6715.

Test Plan: Run the build-linux-aarch64 and build-linux-arm CI builds.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6722