GHA: Fix testing of php versions

The ubuntu-20.04 machine has php 7.4 and 8.0 installed.
configure.ac always looks for newer versions over older versions of php.
To prevent always testing 8.0, remove all versions and just install the
required version.

.github/workflows/ci.yml

@@ -96,7 +96,6 @@ jobs:
         - SWIGLANG: php
           VER: '7.4'
         - SWIGLANG: php
-          VER: '8.0'
         - SWIGLANG: python
         - SWIGLANG: python
           PY3: 3

Tools/CI-linux-install.sh

@@ -83,9 +83,12 @@ case "$SWIGLANG" in
 		$RETRY sudo apt-get -qq install liboctave-dev
 		;;
 	"php")
-		$RETRY sudo add-apt-repository -y ppa:ondrej/php
-		$RETRY sudo apt-get -qq update
-		$RETRY sudo apt-get -qq install php$VER-cli php$VER-dev
+		if [[ "$VER" ]]; then
+			$RETRY sudo apt-get -qq remove "php*-cli" "php*-dev" # Multiple versions are pre-installed
+			$RETRY sudo add-apt-repository -y ppa:ondrej/php
+			$RETRY sudo apt-get -qq update
+			$RETRY sudo apt-get -qq install php$VER-cli php$VER-dev
+		fi
 		;;
 	"python")
 		pip install --user pycodestyle