[ci] Use expansion to test if variables are set
[ci] Use expansion to test if variables are set

Using `-v` test that a variable is set was a feature added in bash 4.2, and MacOS still provides version 3.2, so builds running on mac images cannot include it.

Both versions of bash do support the expansion `${FOO:-bar}` which evaluates to `bar` if `$FOO` is unset, so this can be used to create a compatible test.

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