darwin-install: work around existing vim swapfiles
darwin-install: work around existing vim swapfiles

User on Matrix reported install problems which presented as
"vifs:editing error" which we traced back to vim griping about an
existing swap file. When opened interactively, it did this:

E325: ATTENTION
Found a swap file by the name "/etc/.fstab.swp"
owned by: root dated: Sön Apr 24 16:54:10 2022
file name: /private/etc/fstab
modified: YES
user name: root host name: MBP.local
process ID: 1698
While opening file "/etc/fstab"
dated: Sön Apr 24 16:56:27 2022
NEWER than swap file!
...

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