Revert "NixOS tests: Wait for shell for 10x longer (50m)"

[srhb]

This reverts commit 9bc10e1 from #49441

This commit was used to debug the test failures in especially the internally restarting tests. Tests still experience the same timeout, however, with reduced frequency, which is probably due to the other recent efforts in stabilizing Hydra. Here's a recent failure from installer.zfsroot: https://hydra.nixos.org/build/83682241/nixlog/9/tail

machine# [   25.181616] dhcpcd[1162]: forked to background, child pid 1333
machine# [   25.187133] systemd[1]: Started DHCP Client.
machine# [   25.193277] systemd[1]: Reached target Network.
machine# [   25.197863] systemd[1]: Reached target Network is Online.
machine# [   25.327615] systemd[1]: Starting Permit User Sessions...
machine# [   25.329232] systemd[1]: Started Permit User Sessions.
machine# [   25.330600] systemd[1]: Started Getty on tty1.
machine# [   25.333924] systemd[1]: Reached target Login Prompts.
machine# [   25.337706] systemd[1]: Reached target Multi-User System.
machine# [   25.342149] systemd[1]: Startup finished in 9.169s (kernel) + 16.028s (userspace) = 25.198s.
machine# [   27.843749] dhcpcd[1333]: eth0: no IPv6 Routers available
machine# [  901.130707] systemd[1]: Starting Cleanup of Temporary Directories...
machine# [  901.149885] systemd[1]: Started Cleanup of Temporary Directories.
error: timed out waiting for the VM to connect
timed out waiting for the VM to connect
cleaning up
killing machine (pid 668)
vde_switch: EOF on stdin, cleaning up and exiting
builder for '/nix/store/7csvy3w7g99xd2ghl903zdhw08z2hyk7-vm-test-run-installer-zfs-root.drv' failed with exit code 4

This suggests to me that the increased timeout as suspected doesn't actually do anything, because the failure will occur regardless of how long we wait. If that turns out wrong, we can always re-revert, but I think the root cause should be sought elsewhere within the VM test framework.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

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