Update dfx version

Author: berestovskyy

.github/workflows/rust-simd-example.yaml

@@ -19,10 +19,10 @@ jobs:
     steps:
       - uses: actions/checkout@v1
       - name: Provision Darwin
-        run: DFX_VERSION="0.21.0" bash .github/workflows/provision-darwin.sh
+        run: DFX_VERSION="0.20.2-beta.0" bash .github/workflows/provision-darwin.sh
       - name: Remove networks.json
         run: rm -f ~/.config/dfx/networks.json
-      - name: Rust Periodic Tasks Darwin
+      - name: Rust SIMD Darwin
         run: |
           dfx start --background
           pushd rust/simd
@@ -33,10 +33,10 @@ jobs:
     steps:
       - uses: actions/checkout@v1
       - name: Provision Linux
-        run: DFX_VERSION="0.21.0" bash .github/workflows/provision-linux.sh
+        run: DFX_VERSION="0.20.2-beta.0" bash .github/workflows/provision-linux.sh
       - name: Remove networks.json
         run: rm -f ~/.config/dfx/networks.json
-      - name: Rust Periodic Tasks Linux
+      - name: Rust SIMD Linux
         run: |
           dfx start --background
           pushd rust/simd