ci: catch ram overflow build failures

[sam-golioth]

Twister treats overflows in the RAM during builds as "skipped" tests instead of errors by default. Enabling the --overflow-as-errors option ensures we can catch overflows in CI.

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[sam-golioth]

Great catch!

I'd like to have the same flag enabled in .gitlab-ci.yml, but can do that later myself.

Added!