CI: Consistently use actions/checkout@v2

v2 is the current version of the checkout action and faster than v1.
diff --git a/.github/workflows/musl.yml b/.github/workflows/musl.yml
index c106b1d..aba3f4e 100644
--- a/.github/workflows/musl.yml
+++ b/.github/workflows/musl.yml
@@ -13,7 +13,7 @@
       container:
         image: alpine:latest
       steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Install dependencies
         run: apk add gcc make tar git python3 libc-dev linux-headers pcre-dev pcre2-dev openssl-dev lua5.3-dev grep socat curl
       - name: Install VTest