use existing GID in sequoia migration script

[abathur]

Motivation

I hardcoded the wrong GID (30001 instead of 30000), but it's better to just pick up the GID from the existing group.

cc @tomberek

Context

• install-darwin: fix _nixbld uids for macOS sequoia #10919 (comment)
• install-darwin: fix _nixbld uids for macOS sequoia #10919 (comment)

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[tomberek]

I believe -create will not change an existing user's Primary GID. It may require a -change call.

[abathur]

In this case, the function's just running to re-create the missing _nixbldN users clobbered by the update.

That said, though, we have been tending to use -create (over -change) since we've generally observed it to be ~idempotent as long as we aren't trying to do something like change the UID to one that's already taken:

$ sudo /usr/bin/dscl . -read "/Users/_nixbld1" "PrimaryGroupID"
PrimaryGroupID: 350

$ sudo /usr/bin/dscl . -create "/Users/_nixbld1" "PrimaryGroupID" "351"

$ sudo /usr/bin/dscl . -read "/Users/_nixbld1" "PrimaryGroupID"
PrimaryGroupID: 351

[tomberek]

I was wrong... this should update the users. Thanks!