Require that SELECT_XCODE is set.

Allowing the Xcode version to "float" based on whatever default GitHub
selects creates an unreliable environment. When GitHub changes the
default, we can have multiple jobs in the same run using different
versions as it rolls out across machines. It can also cause oscillation
between runs as different machines are used. It also causes
unpredictable timing when the updates happen.

This change helps ensure that the version that is used is pinned. The
downside is that it requires manually bumping the version, and the risk
that if we take too long, older Xcodes will be removed and that will
break the build.

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