ci: keep Linux self-tests on established compiler

Author: medvednikov

ci/linux_ci.vsh

@@ -20,11 +20,22 @@ fn test_pure_v_math_module() {
 	exec('v -exclude @vlib/math/*.c.v test vlib/math/')
 }
 
+fn use_established_compiler_for_self_tests() {
+	current_vflags := os.getenv('VFLAGS')
+	if '-old-compiler' !in current_vflags.fields() {
+		os.setenv('VFLAGS', '${current_vflags} -old-compiler'.trim_space(), true)
+	}
+}
+
 fn self_tests() {
+	// The dedicated V3 workflow owns strict V3 coverage. Keep the full vlib suite
+	// and its nested compiler invocations on V1 so the compatibility compiler
+	// shipped by this rollout remains tested.
+	use_established_compiler_for_self_tests()
 	if common.is_github_job {
-		exec('v -W -silent test-self vlib')
+		exec('v -old-compiler -W -silent test-self vlib')
 	} else {
-		exec('v -progress test-self vlib')
+		exec('v -old-compiler -progress test-self vlib')
 	}
 }
 
@@ -242,12 +253,14 @@ fn self_tests_gcc() {
 }
 
 fn self_tests_prod_gcc() {
+	use_established_compiler_for_self_tests()
 	exec('v -o vprod -prod cmd/v')
-	exec('./vprod -silent test-self vlib')
+	exec('./vprod -old-compiler -silent test-self vlib')
 }
 
 fn self_tests_cstrict_gcc() {
-	exec('VTEST_JUST_ESSENTIAL=1 V_CI_CSTRICT=1 v -cc gcc -cstrict -silent test-self vlib')
+	use_established_compiler_for_self_tests()
+	exec('VTEST_JUST_ESSENTIAL=1 V_CI_CSTRICT=1 v -old-compiler -cc gcc -cstrict -silent test-self vlib')
 }
 
 fn build_examples_gcc() {
@@ -366,12 +379,14 @@ fn self_tests_clang() {
 }
 
 fn self_tests_vprod_clang() {
+	use_established_compiler_for_self_tests()
 	exec('v -o vprod -prod cmd/v')
-	exec('./vprod -silent test-self vlib')
+	exec('./vprod -old-compiler -silent test-self vlib')
 }
 
 fn self_tests_cstrict_clang() {
-	exec('VTEST_JUST_ESSENTIAL=1 V_CI_CSTRICT=1 ./vprod -cstrict -silent test-self vlib')
+	use_established_compiler_for_self_tests()
+	exec('VTEST_JUST_ESSENTIAL=1 V_CI_CSTRICT=1 ./vprod -old-compiler -cstrict -silent test-self vlib')
 }
 
 fn build_examples_clang() {