Install cargo-insta in dev containers (#909)

Since cargo-insta is a dependency to manage tests, the dev environment
is incomplete if it is not installed.

.devcontainer/devcontainer.json

@@ -4,7 +4,7 @@
     "dockerfile": "Dockerfile",
     "args": { "VARIANT": "buster" }
   },
-  "postCreateCommand": "git config --global --add safe.directory $PWD",
+  "postCreateCommand": "git config --global --add safe.directory $PWD && cargo install cargo-insta",
   "extensions": ["EditorConfig.EditorConfig"],
   "remoteUser": "vscode"
 }

.gitpod.yml

@@ -5,6 +5,8 @@ vscode:
     - 'rust-lang.rust-analyzer'
     - 'tamasfe.even-better-toml'
 tasks:
+  - name: Install cargo-insta
+    init: cargo install cargo-insta
   - name: Build Onefetch
     before: sudo apt install -y cmake
     init: cargo build