build: Use env instead of TestEnvironment key

We rely on having the DISPLAY environment variable set, otherwise we
default to skipping all tests automatically. The TestEnvironment key
inside the installed test launcher keyfile replaces the whole
environment, instead of just adding to it like the TESTS_ENVIRONMENT
automake variable.

build/autotools/glib-tap.mk

@@ -127,8 +127,7 @@ installed_test_meta_DATA = $(installed_testcases:=.test)
 %.test: %$(EXEEXT) Makefile
 	$(AM_V_GEN) (echo '[Test]' > $@.tmp; \
 	echo 'Type=session' >> $@.tmp; \
-	echo 'TestEnvironment=G_ENABLE_DIAGNOSTIC=0;CLUTTER_ENABLE_DIAGNOSTIC=0;' >> $@.tmp; \
-	echo 'Exec=$(installed_testdir)/$<' >> $@.tmp; \
+	echo 'Exec=env G_ENABLE_DIAGNOSTIC=0 CLUTTER_ENABLE_DIAGNOSTIC=0 $(installed_testdir)/$<' >> $@.tmp; \
 	mv $@.tmp $@)
 
 CLEANFILES += $(installed_test_meta_DATA)