gh-38067: `.ci/write-dockerfile.sh`: Quoting fix for `unminimize`
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On docker-ubuntu, we attempted to remove the `unminimize` program after
running it once, but this has failed because of quoting mistake. https:
//github.com/mkoeppe/sage/actions/runs/9219405769/job/25364502464#step:9
:311
Fixed here.
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URL: https://github.com/sagemath/sage/pull/38067
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe
