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

[mergify]

Motivation

Starting in macOS 15 Sequoia, macOS daemon UIDs are encroaching on our default UIDs of 301-332. This commit relocates our range up to avoid clashing with the current UIDs of 301-304 and buy us a little time while still leaving headroom for people installing more than 32 users.

It also adopts GID 350 (same as first UID), since @emilazy pointed out that this will keep our build group from showing up in the Users & Groups interface. (See #10919 (comment))

Context

• macOS 15 Sequoia clobbers _nixbld1-4 users #10892
• _nixbld 1 through 4 have disappeared #10912
• fix nixbld user name/uid for macOS/darwin #4532
• macOS update "booted to Recovery" because "an error occurred migrating user data" on Big Sur/Monterey w/ multi-user Nix present  #4531

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This is an automatic backport of pull request #10919 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of df36ff0 has failed:

On branch mergify/bp/2.19-maintenance/pr-10919
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You are currently cherry-picking commit df36ff0d1.
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Changes to be committed:
	modified:   scripts/bigsur-nixbld-user-migration.sh

Unmerged paths:
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	both modified:   scripts/install-darwin-multi-user.sh

Cherry-pick of 7556742 has failed:

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Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   scripts/install-multi-user.sh
	both modified:   scripts/install-systemd-multi-user.sh

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[emilazy]

I believe backporting #9639 would resolve the conflict here.

[tomberek]

@Mergifyio rebase

[mergify]

rebase

☑️ Nothing to do

• -conflict [📌 rebase requirement]
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• queue-position=-1 [📌 rebase requirement]
• any of:
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