Fix undefined variable in install_from_external_spec error message (apache#63233)

The error message in install_from_external_spec() referenced
${INSTALLATION_METHOD} which does not exist — the correct variable
is ${AIRFLOW_INSTALLATION_METHOD}. With set -u active, hitting this
error path would crash with an "unbound variable" error instead of
printing the intended user-friendly message.

The typo was introduced in a1717a6 and carried forward into the
inlined copies in both Dockerfiles.

Author: XD-DENG

Dockerfile

@@ -1201,7 +1201,7 @@ function install_from_external_spec() {
         installation_command_flags="apache-airflow[${AIRFLOW_EXTRAS}]${AIRFLOW_VERSION_SPECIFICATION}"
     else
         echo
-        echo "${COLOR_RED}The '${INSTALLATION_METHOD}' installation method is not supported${COLOR_RESET}"
+        echo "${COLOR_RED}The '${AIRFLOW_INSTALLATION_METHOD}' installation method is not supported${COLOR_RESET}"
         echo
         echo "${COLOR_YELLOW}Supported methods are ('.', 'apache-airflow')${COLOR_RESET}"
         echo

Dockerfile.ci

@@ -936,7 +936,7 @@ function install_from_external_spec() {
         installation_command_flags="apache-airflow[${AIRFLOW_EXTRAS}]${AIRFLOW_VERSION_SPECIFICATION}"
     else
         echo
-        echo "${COLOR_RED}The '${INSTALLATION_METHOD}' installation method is not supported${COLOR_RESET}"
+        echo "${COLOR_RED}The '${AIRFLOW_INSTALLATION_METHOD}' installation method is not supported${COLOR_RESET}"
         echo
         echo "${COLOR_YELLOW}Supported methods are ('.', 'apache-airflow')${COLOR_RESET}"
         echo

scripts/docker/install_airflow_when_building_images.sh

@@ -148,7 +148,7 @@ function install_from_external_spec() {
         installation_command_flags="apache-airflow[${AIRFLOW_EXTRAS}]${AIRFLOW_VERSION_SPECIFICATION}"
     else
         echo
-        echo "${COLOR_RED}The '${INSTALLATION_METHOD}' installation method is not supported${COLOR_RESET}"
+        echo "${COLOR_RED}The '${AIRFLOW_INSTALLATION_METHOD}' installation method is not supported${COLOR_RESET}"
         echo
         echo "${COLOR_YELLOW}Supported methods are ('.', 'apache-airflow')${COLOR_RESET}"
         echo