GH Actions: split off Psalm to separate workflow

Psalm does not need to be run against multiple PHP versions. Running it once should be enough.

With that in mind, this commit:
* Introduces a separate, dedicated workflow which only installs and runs Psalm.
* Removes the Psalm related steps from the `CI` workflow.
* Removes Psalm from the `tools` setting for `setup-php`.

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl
           ini-values: post_max_size=256M, max_execution_time=180
-          tools: psalm, phpunit:${{ matrix.phpunit-versions }}
+          tools: phpunit:${{ matrix.phpunit-versions }}
           coverage: none
 
       - name: Use Composer 1.x
@@ -53,7 +53,7 @@ jobs:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl, sodium
           ini-values: post_max_size=256M, max_execution_time=180
-          tools: psalm, phpunit:${{ matrix.phpunit-versions }}
+          tools: phpunit:${{ matrix.phpunit-versions }}
           coverage: none
 
       - name: Modernize dependencies
@@ -86,7 +86,7 @@ jobs:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl, sodium
           ini-values: post_max_size=256M, max_execution_time=180
-          tools: psalm, phpunit:${{ matrix.phpunit-versions }}
+          tools: phpunit:${{ matrix.phpunit-versions }}
           coverage: none
 
       - name: Modernize dependencies
@@ -100,9 +100,3 @@ jobs:
         timeout-minutes: 30
         with:
           memory_limit: 256M
-
-      - name: Install Psalm
-        run: rm composer.lock && composer require --dev vimeo/psalm:^4
-
-      - name: Static Analysis
-        run: vendor/bin/psalm

.github/workflows/psalm.yml

@@ -0,0 +1,30 @@
+name: Psalm
+
+on: [push]
+
+jobs:
+  psalm:
+    name: Psalm on PHP ${{ matrix.php-versions }}
+    runs-on: ${{ matrix.operating-system }}
+    strategy:
+      matrix:
+        operating-system: ['ubuntu-latest']
+        php-versions: ['7.4']
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+          tools: psalm:4
+          coverage: none
+
+      - name: Install Composer dependencies
+        uses: "ramsey/composer-install@v1"
+        with:
+          composer-options: --no-dev
+
+      - name: Static Analysis
+        run: psalm