action: Lock rust version to 1.52

Got the following error in the clippy:
error: this expression borrows a reference (`&str`) that is immediately dereferenced by the compiler
  --> tests/cpuset.rs:54:37
   |
54 |             let r = cpuset.set_cpus(&cpus);
   |                                     ^^^^^ help: change this to: `cpus`
   |
   = note: `-D clippy::needless-borrow` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

But we did not touch the code, just becuase the clippy rules was updated
follow the rust update.

To avoid this problem we lock the rust version in CI and make it
steady.

Signed-off-by: Tim Zhang <tim@hyper.sh>

.github/workflows/bvt.yaml

@@ -1,20 +1,22 @@
 name: BVT
 on: [pull_request]
+env:
+  RUST_VERSION: 1.52
 jobs:
   build:
     name: Build
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout
-        uses: actions/checkout@v2
-      - name: Build
-        run: make debug
+      - uses: actions/checkout@v2
+      - run: rustup install ${{ env.RUST_VERSION }} && rustup default ${{ env.RUST_VERSION }}
+      - run: make debug
 
   fmt:
     name: Format Check
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - run: rustup install ${{ env.RUST_VERSION }} && rustup default ${{ env.RUST_VERSION }}
       - run: rustup component add rustfmt
       - run: make fmt
   clippy:
@@ -22,6 +24,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - run: rustup install ${{ env.RUST_VERSION }} && rustup default ${{ env.RUST_VERSION }}
       - run: rustup component add clippy
       - run: make clippy
   test:
@@ -29,6 +32,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - run: rustup install ${{ env.RUST_VERSION }} && rustup default ${{ env.RUST_VERSION }}
       - run: make test