nixosTests.acme: Use a read-only pkgs

This speeds up evaluation by a factor 2.

Ballpark figures from my machine:

```
$ time nix-build nixos/release.nix -A tests.acme
/nix/store/q4fxp55k64clcarsx8xc8f6s10szlfvz-vm-test-run-acme
/nix/store/lnfqg051sxx05hclva84bcbnjfc71c8x-vm-test-run-acme

real 1m28.142s
user 1m7.474s
sys 0m7.932s

$ time nix-build nixos/release.nix -A tests.acme
/nix/store/q4fxp55k64clcarsx8xc8f6s10szlfvz-vm-test-run-acme
/nix/store/lnfqg051sxx05hclva84bcbnjfc71c8x-vm-test-run-acme

real 0m38.235s
user 0m33.814s
sys 0m2.283s

```

Changed files
+1 -1
nixos
+1 -1
nixos/tests/all-tests.nix
···
_3proxy = runTest ./3proxy.nix;
aaaaxy = runTest ./aaaaxy.nix;
-
acme = runTest ./acme.nix;
+
acme = runTest { imports = [ ./acme.nix readOnlyPkgs ]; };
adguardhome = runTest ./adguardhome.nix;
aesmd = runTestOn ["x86_64-linux"] ./aesmd.nix;
agate = runTest ./web-servers/agate.nix;