nixos/tests: Fix wrong arch in runInMachine test

Since 83b27f60

, the tests were moved
into all-tests.nix and some of the tooling has changed so that
subattributes of test expressions are now recursively evaluated until a
derivation with a .test attribute has been found.

Unfortunately this isn't the case for all of the tests and the
runInMachine doesn't use the makeTest function other tests are using but
instead uses runInMachine, which doesn't generate a .test attribute.

Whener a .test attribute wasn't found by the new handleTest function, it
recurses down again until there is no value left that is an attribute
set and subsequently returns its unchanged value. This however has the
drawback that instead of getting different attributes for each
architecture we only get the last architecture in the supportedSystems
list.

In the case of the release.nix, the last architecture in
supportedSystems is "aarch64-linux", so the runInMachine test is always
built on that architecture.

In order to work around this, I changed runInMachine to emit a .test
attribute so that it looks to handleTest like it was a test created via
makeTest.

Signed-off-by: aszlig <aszlig@nix.build>