build1 for Tor Browser 8.5a9 is broken in mobile packaging step

When packaging mobile Firefox an error occurs:

0:00.63 fa:
 0:00.63 changed: 2104
 0:00.63 changed_w: 10959
 0:00.63 keys: 41
 0:00.63 missing: 23
 0:00.63 missingInFiles: 48
 0:00.63 missing_w: 580
 0:00.63 obsolete: 23
 0:00.63 unchanged: 87
 0:00.63 unchanged_w: 102
 0:00.63 93% of entries changed
 0:00.69 \nGenerating: search-jar
 0:00.76 Traceback (most recent call last):
 0:00.76   File "/usr/lib/python2.7/runpy.py", line 174, in _run_module_as_main
 0:00.76     "__main__", fname, loader, pkg_name)
 0:00.76   File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
 0:00.76     exec code in run_globals
 0:00.76   File "/var/tmp/build/firefox-16cd9b21911c/python/mozbuild/mozbuild/action/jar_maker.py", line 17, in <module>
 0:00.76     sys.exit(main(sys.argv[1:]))
 0:00.76   File "/var/tmp/build/firefox-16cd9b21911c/python/mozbuild/mozbuild/action/jar_maker.py", line 13, in main
 0:00.76     return mozbuild.jar.main(args)
 0:00.76   File "/var/tmp/build/firefox-16cd9b21911c/python/mozbuild/mozbuild/jar.py", line 608, in main
 0:00.76     jm.makeJar(infile, options.d)
 0:00.76   File "/var/tmp/build/firefox-16cd9b21911c/python/mozbuild/mozbuild/jar.py", line 333, in makeJar
 0:00.76     self.processJarSection(info, jardir)
 0:00.76   File "/var/tmp/build/firefox-16cd9b21911c/python/mozbuild/mozbuild/jar.py", line 388, in processJarSection
 0:00.76     self._processEntryLine(e, outHelper, jf)
 0:00.76   File "/var/tmp/build/firefox-16cd9b21911c/python/mozbuild/mozbuild/jar.py", line 460, in _processEntryLine
 0:00.76     ', '.join(src_base)))
 0:00.76 RuntimeError: File "google-b-m.xml" not found in /var/tmp/build/firefox-16cd9b21911c/mobile/locales/searchplugins, /var/tmp/build/firefox-16cd9b21911c/obj-arm-linux-androideabi/mobile/locales/.deps/generated_fa, /var/tmp/build/firefox-16cd9b21911c/obj-arm-linux-androideabi/mobile/locales/.deps/generated_fa, /var/tmp/build/firefox-16cd9b21911c/obj-arm-linux-androideabi/mobile/locales
 0:00.76 Makefile:117: recipe for target 'searchplugins' failed
 0:00.76 make[3]: *** [searchplugins] Error 1
 0:00.76 Makefile:144: recipe for target 'chrome-fa' failed
 0:00.76 make[2]: *** [chrome-fa] Error 2
 0:00.76 Makefile:66: recipe for target 'chrome-fa' failed
 0:00.76 make[1]: *** [chrome-fa] Error 2
 0:00.76 /var/tmp/build/firefox-16cd9b21911c/mobile/android/build.mk:60: recipe for target 'chrome-fa' failed
 0:00.76 make: *** [chrome-fa] Error 2

That's due to https://bugzilla.mozilla.org/show_bug.cgi?id=1510296 backporting a search engine related patch that changes the google search engine on mobile from google to google-b-m without providing a respective .xml file.

And as Mozilla does not build Firefox for mobile on ESR 60 this remained undetected.