Build persistent store with frozen compiler

But still make sure it passes tests with the most recent nightly.

.github/workflows/persistent_store_test.yml

@@ -12,20 +12,33 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
+
+      - uses: actions-rs/toolchain@v1
+        with:
+          target: thumbv7em-none-eabi
 
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: nightly
-          override: true
+
+      - name: Build Persistent store library
+        uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --manifest-path libraries/persistent_store/Cargo.toml --release --target=thumbv7em-none-eabi
 
       - name: Unit testing of Persistent store library (release mode)
         uses: actions-rs/cargo@v1
         with:
+          toolchain: nightly
           command: test
-          args: --manifest-path libraries/persistent_store/Cargo.toml --release --features std
+          args: --manifest-path libraries/persistent_store/Cargo.toml --release --features=std
 
       - name: Unit testing of Persistent store library (debug mode)
         uses: actions-rs/cargo@v1
         with:
+          toolchain: nightly
           command: test
-          args: --manifest-path libraries/persistent_store/Cargo.toml --features std
+          args: --manifest-path libraries/persistent_store/Cargo.toml --features=std