test: Don't mount build sources into image when running non-interactively

.github/workflows/mkosi.yml

@@ -159,8 +159,6 @@ jobs:
 
         [Host]
         QemuMem=4G
-        # We build with debuginfo so there's no point in mounting the sources into the machine.
-        RuntimeBuildSources=no
         EOF
 
     - name: Generate secure boot key

test/integration-test-wrapper.py

@@ -154,6 +154,7 @@ def main():
             ),
         ]),
         '--credential', f"journal.storage={'persistent' if sys.stderr.isatty() else args.storage}",
+        *(['--runtime-build-sources=no'] if not sys.stderr.isatty() else []),
         'qemu' if args.vm or os.getuid() != 0 else 'boot',
     ]