[CI] Use proper argument handling in build_cmake.sh
[CI] Use proper argument handling in build_cmake.sh

Summary:
D6315 was branching on `$1` to check for an option but will fail due to
unbound variable (`set -u`) if no argument is set.
This diffs use proper handling to avoid the issue and make later
improvements easier.

Test Plan:
Run the build-clang-10 CI build, or any other using the `--Werror`
option.
Run the build-osx CI build, or any other not using the `--Werror`
option.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6325

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