makefile: mkdir is not needed on templatized docker builds
This imitates
e63dfbbdbd07 and is a small addition to
231690dba9b4 and
5c1283713293
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
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273ce12ad8f1` will report some style violations on a very old commit).