Fix cargo PATH: use explicit export in every step that needs it

The Gitea Act runner's Docker container does not reliably support
$GITHUB_PATH or sourcing ~/.cargo/env across steps. Both mechanisms
failed because the runner spawns a fresh shell for each step.

Adopted the same pattern that already works for the Windows job:
explicitly set PATH at the top of every step that calls cargo or
npx tauri. This is the most portable approach across all runner
environments (Act Docker containers, bare metal macOS, Windows).

Build history shows Linux succeeded through run#46 (JS-based
actions) and failed from run#49 onward (shell-based installs).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build-app.yml

@@ -81,7 +81,7 @@ jobs:
           else
             curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
           fi
-          echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+          export PATH="$HOME/.cargo/bin:$PATH"
           rustc --version
           cargo --version
 
@@ -91,11 +91,15 @@ jobs:
 
       - name: Install Tauri CLI
         working-directory: ./app
-        run: npx tauri --version || npm install @tauri-apps/cli
+        run: |
+          export PATH="$HOME/.cargo/bin:$PATH"
+          npx tauri --version || npm install @tauri-apps/cli
 
       - name: Build Tauri app
         working-directory: ./app
-        run: npx tauri build
+        run: |
+          export PATH="$HOME/.cargo/bin:$PATH"
+          npx tauri build
 
       - name: Collect artifacts
         run: |
@@ -136,7 +140,7 @@ jobs:
     steps:
       - name: Install Node.js
         run: |
-          if command -v node &>/dev/null; then
+          if command -v node >/dev/null 2>&1; then
             echo "Node.js already installed: $(node --version)"
           else
             echo "Installing Node.js 22 via Homebrew..."
@@ -176,7 +180,7 @@ jobs:
           else
             curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
           fi
-          echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+          export PATH="$HOME/.cargo/bin:$PATH"
           rustup target add aarch64-apple-darwin x86_64-apple-darwin
           rustc --version
           cargo --version
@@ -187,11 +191,15 @@ jobs:
 
       - name: Install Tauri CLI
         working-directory: ./app
-        run: npx tauri --version || npm install @tauri-apps/cli
+        run: |
+          export PATH="$HOME/.cargo/bin:$PATH"
+          npx tauri --version || npm install @tauri-apps/cli
 
       - name: Build Tauri app (universal)
         working-directory: ./app
-        run: npx tauri build --target universal-apple-darwin
+        run: |
+          export PATH="$HOME/.cargo/bin:$PATH"
+          npx tauri build --target universal-apple-darwin
 
       - name: Collect artifacts
         run: |