Fix broken CI due to unwanted PHP update

Force specific PHP version for all necessary CI steps.

.github/workflows/ci.yaml

@@ -34,6 +34,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: "7.3"
+
       - name: Get Composer Cache Directory
         id: composer-cache
         run: |
@@ -61,6 +66,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: "7.3"
+
       - name: Install xmllint
         run: sudo apt-get install libxml2-utils
 
@@ -102,6 +112,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: "7.3"
+
       - name: Get Composer Cache Directory
         id: composer-cache
         run: |
@@ -216,6 +231,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: "7.3"
+
       - name: Get Composer Cache Directory
         id: composer-cache
         run: |