Allow to override toolchain from rust-toolchain file (#35)

.github/workflows/ci.yml

@@ -1,9 +1,9 @@
 name: Continuous integration
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
-  check_pr:
+  test:
     runs-on: ubuntu-latest
     steps:
       - name: Create npm configuration
@@ -18,7 +18,6 @@ jobs:
 
   install_stable:
     runs-on: ubuntu-latest
-    needs: check_pr
     steps:
       - uses: actions/checkout@v1
       - uses: ./
@@ -27,7 +26,6 @@ jobs:
 
   install_nightly:
     runs-on: ubuntu-latest
-    needs: check_pr
     steps:
       - uses: actions/checkout@v1
       - uses: ./
@@ -39,7 +37,6 @@ jobs:
   install_stable_in_docker:
     runs-on: ubuntu-latest
     container: ubuntu:latest  # Docker image, not the GitHub Actions VM
-    needs: check_pr
     steps:
       # `rustup` will need `curl` or `wget` later
       - run: apt-get update && apt-get install -y curl
@@ -47,3 +44,10 @@ jobs:
       - uses: ./
         with:
           toolchain: stable
+
+  install_stable_through_rust_toolchain_file:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - run: echo "stable" > ./rust-toolchain
+      - uses: ./