hadrian: fix `test --test-compiler=stage1` (and making sure it stays that way)
I want to run the testsuite with the stage1 compiler, because at the moment, the stage2 compiler on my branch crashes.
So I do
$ hadrian/build --flavour=validate test --test-compiler=stage1
...
Unknown program "_validate/stage0/bin/runghc"
Build failed.
which is worse than last time I tried, when I remembered getting an error at some later point (but still before running a single test). We should make sure that running the testsuite is possible even if the stage2 compiler is broken, perhaps by testing hadrian/build test --test-compiler=stage1 --only=""
in CI.