nixos/tests/installer.nix: shorten test name to fit length limit

previously one of the tests would fail with

boot-after-install: starting vm
Path length (109) is longer than maximum supported length (108) and will be truncated at /nix/store/0ingn8cwwnl84i374hcl6nafsm2c5m2p-perl-5.28.0/lib/perl5/5.28.0/x86_64-linux-thread-multi/Socket.pm line 872, <__ANONIO__> line 268.
boot-after-install# qemu-system-x86_64: -monitor unix:./monitor: Failed to connect socket ./monitor: No such file or directory
error: QEMU died prematurely
QEMU died prematurely
vde_switch: EOF on stdin, cleaning up and exiting
cleaning up
builder for '/nix/store/zbpxwwxwy7f6z5z3kg4nf5mjqsywzjvx-vm-test-run-installer-filesystemEncryptedWithKeyfile.drv' failed with exit code 4

Shortening the test name fixes the issue.