chore(actions): fix tty device being nonexistent in test step

`/dev/tty` does not exist in an actions step since it is being run non
interactively. Similarly using the stdout device does not work either,
since we're within a command substitution expression, where `/dev/stdout`
is not attached to a shell.

Instead, we have to use `/dev/stderr`, which is attached to a shell at
all times. I don't know if there are any better alternatives, but
writing to stderr doesn't sit quite right with me, since this should
ideally output to stdout. Oh, well.

.github/workflows/ci.yml

@@ -118,5 +118,5 @@ jobs:
         run: |
           # HACK: Since the exit code isn't properly reflected due to frktest not returning
           # the correct status on test fails, we have to parse the output and check ourselves
-          output="$(nix develop -c lune run tests extract | tee /dev/tty)"
+          output="$(nix develop -c lune run tests extract | tee /dev/stderr)"
           tail -n 1 <<< $output | sed 's/\x1b\[[0-9;]*m//g' | grep -q "status: OK"