test(agc): keep the retire test's mock device out of the Linux import probe

On Linux with write watching (uffd write-protect), the first import asks the device whether a GPU
write clears the protection (ProbeImportWriteProtection): it records a vkCmdCopyBuffer and a
vkCmdFillBuffer on the import and submits them. The test's mock device records no commands, so
agc_host_import_retire failed with "missing Vulkan function: vkCmdCopyBuffer" before it reached
the retire (RADV, reported on #4515). The test now sets APS5_WRITE_WATCH_IMPORTS=unwatch, so its
imports are compared as on Windows and the probe, which a mock cannot answer, does not run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/libs/prx/libSceAgcDriver/tests/HostImportRetireHold.cpp

@@ -331,6 +331,7 @@ int main() {
     _putenv_s("APS5_RELEASE_ON_UNLOCK", "1");
 #else
     setenv("APS5_RELEASE_ON_UNLOCK", "1", 1);
+    setenv("APS5_WRITE_WATCH_IMPORTS", "unwatch", 1);
 #endif
     try {
         IdleRetireOutlivesTheHold();