verify msrv in ci

.github/workflows/rust.yml

@@ -147,4 +147,21 @@ jobs:
       - uses: actions/upload-artifact@v2
         with:
           name: cargo-espflash.exe
-          path: target/x86_64-pc-windows-gnu/release/cargo-espflash.exe
+          path: target/x86_64-pc-windows-gnu/release/cargo-espflash.exe
+  msrv:
+    name: Check MSRV
+    runs-on: ubuntu-latest
+    steps:
+      - name: build dependencies
+        run: |
+          sudo apt-get install musl-tools libudev-dev
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: 1.55
+          override: true
+      - uses: Swatinem/rust-cache@v1
+      - uses: actions-rs/cargo@v1
+        with:
+          command: check