Use dfx with gzip support for defi examples

CI was broken for motoko and rust defi examples because the `dfx`
version used could not handle the gzipped II canister.

Author: Frederik Rothenberger

.github/workflows/motoko-defi-example.yml

@@ -21,6 +21,8 @@ jobs:
         with:
           submodules: recursive
       - name: Provision Darwin
+        env:
+          DFX_VERSION: 0.15.3
         run: bash .github/workflows/provision-darwin.sh
       - name: Motoko Defi Darwin
         run: |

.github/workflows/rust-defi-example.yml

@@ -21,6 +21,8 @@ jobs:
         with:
           submodules: recursive
       - name: Provision Darwin
+        env:
+          DFX_VERSION: 0.15.3
         run: bash .github/workflows/provision-darwin.sh
       - name: Rust Defi Darwin
         run: |