[secp256k1] ci: fix env vars and set -u to treat unset variables as errors

Summary:

• ci: print env to allow reproducing the job outside of CI
• Set -u to treat unset variables as an error
• We can get rid of $USE_HOST by specifying --host="$HOST" directly.
• Fix inconsistencies between CTIMETEST and CTIMETESTS (backport https://github.com/bitcoin-core/secp256k1/pull/1169/changes/0f088ec11263261497661215c110a4c395acc0ac)
• Disable CTIMETESTS for MSAN, as this was enabled prematurely in D19527 (more backports needed)
• RUN_VALGRIND no longer exists since D12960 and CIRRUS_OS is now RUNNER_OS since we migrated to github actions
• Make sure CMAKE_EXTRA_FLAGS is defined at the end of build_cmake.sh, and fix array appending (don't do string concat, which results in missing spaces)

This is a backport of secp256k1#779, secp256k1#1147 and secp256k1#1154

Test Plan: push to github and run CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19572