Force LC_ALL=C.UTF-8 during build

So that builds in non-English locales can pass their build-time tests.

debian/rules

@@ -2,6 +2,8 @@
 
 export DEB_BUILD_MAINT_OPTIONS = hardening=+all
 export DEB_LDFLAGS_MAINT_APPEND = -Wl,--as-needed
+# Some tests assert that we will see English strings
+export LC_ALL=C.UTF-8
 
 %:
 	dh $@ --with=gir