Queue rather than cancel concurrent macOS signing builds

Copied from the QA builds without thinking, where cancelling an in-flight run
on a new push is exactly right: the tip is what matters and a superseded build
is wasted effort.

A signing build is not that. Cancelling one throws away forty minutes of work,
and worse, it can abandon a notarisation already submitted to Apple, leaving a
request in their queue for a build nobody will ever collect. The Windows
snapshot already queues rather than cancels, for the related reason that there
is one hardware token, so this brings the two into line.

Found by cancelling my own in-flight Intel notarisation with an unrelated push.

.github/workflows/snapshot-macos.yml

@@ -29,8 +29,12 @@ on:
       - 'pkg/mac/**'
 
 concurrency:
+  # Not cancel-in-progress, unlike the QA builds. Cancelling a signing build
+  # partway through wastes forty minutes of work and, worse, can abandon a
+  # notarisation already submitted to Apple, leaving a request sitting in their
+  # queue for a build nobody will collect. Queue instead.
   group: '${{ github.workflow }} @ ${{ github.ref }}'
-  cancel-in-progress: true
+  cancel-in-progress: false
 
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   contents: read