ci: pass V_CI_MUSL=1 on musl CI jobs

.github/workflows/ci.yml

@@ -43,6 +43,8 @@ jobs:
   alpine-docker-musl-gcc:
     name: alpine-musl
     runs-on: ubuntu-latest
+    env:
+      V_CI_MUSL: 1
     steps:
 
     - name: Checkout
@@ -208,6 +210,7 @@ jobs:
     runs-on: ubuntu-18.04
     env:
       VFLAGS: -cc musl-gcc
+      V_CI_MUSL: 1
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-node@v1

vlib/v/tests/valgrind/valgrind_test.v

@@ -12,6 +12,10 @@ fn test_all() {
 		eprintln('You can still do it by setting FORCE_VALGRIND_TEST=1 .')
 		exit(0)
 	}
+	if os.getenv('V_CI_MUSL').len > 0 {
+		eprintln('This test is disabled for musl.')
+		exit(0)
+	}
 	bench_message := 'memory leak checking with valgrind'
 	mut bench := benchmark.new_benchmark()
 	eprintln(term.header(bench_message, '-'))