refactor(run.sh): unify calling of python

* `/usr/bin/env` is not necessary: `python3` is just the same
  (`/usr/bin/env python3` is helpful in a shebang, since an absolute
  path is required there)
* use the variable `$PYTHON_EXEC`: it was introduced recently, but not
  used for now

run.sh

@@ -34,7 +34,7 @@ echo "[ Docker=$docker ]"
 
 # Check for the correct Python version (3.8+)
 PYTHON_EXEC="python3"
-echo "[ Using $(python3 -V) ]"
+echo "[ Using $("$PYTHON_EXEC" -V) ]"
 
 
 # +++ DEFAULT COMMANDS +++ #
@@ -45,24 +45,24 @@ if [[ $docker == "false" ]]; then
 
 	if [[ $mode == "dev" ]]; then
 		source python/bin/activate
-		echo "Running Engine API in Python Environment ($(python3 -V))"
+		echo "Running Engine API in Python Environment ($("$PYTHON_EXEC" -V))"
 		echo "Starting API Server"
-		python src/app.py
+		"$PYTHON_EXEC" src/app.py
 	fi
 
 	### Runs the API Server (Test) ###
 
 	if [[ $mode == "api-test-0" ]]; then
 		echo "Starting API Server 0"
-		/usr/bin/env python3 src/app.py config=tests/config/engine-0-api.ini
+		"$PYTHON_EXEC" src/app.py config=tests/config/engine-0-api.ini
 	fi
 	if [[ $mode == "api-test-1" ]]; then
 		echo "Starting API Server 1"
-		/usr/bin/env python3 src/app.py config=tests/config/engine-1-api.ini
+		"$PYTHON_EXEC" src/app.py config=tests/config/engine-1-api.ini
 	fi
 	if [[ $mode == "api-test-2" ]]; then
 		echo "Starting API Server 2"
-		/usr/bin/env python3 src/app.py config=tests/config/engine-2-api.ini
+		"$PYTHON_EXEC" src/app.py config=tests/config/engine-2-api.ini
 	fi
 
 	### Runs the API Server using Gunicorn without a system daemon (Production) ###
@@ -80,7 +80,7 @@ if [[ $docker == "false" ]]; then
 	### CAUTION: This deletes everything in your database ###
 
 	if [[ $mode == "recreate-database" ]]; then
-		/usr/bin/env python3 src/app.py --recreate-database
+		"$PYTHON_EXEC" src/app.py --recreate-database
 	fi
 
 fi
@@ -117,4 +117,4 @@ if [[ $docker == "true" ]]; then
 	if [[ $mode == "push" ]]; then
 		exec sudo docker push autoradio/engine-api
 	fi
-fi
+fi
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