Simplify our "does this image contain the expected Python" test

test/tests/python-hy/container.sh

@@ -3,8 +3,8 @@ set -e
 
 python=
 for c in pypy3 pypy python3 python; do
-	if PATH=/usr/local/bin command -v "$c" > /dev/null; then
-		python="$c"
+	if [ -x "/usr/local/bin/$c" ]; then
+		python="/usr/local/bin/$c"
 		break
 	fi
 done

test/tests/run-python-in-container.sh

@@ -6,8 +6,8 @@ runDir="$(dirname "$(readlink -f "$BASH_SOURCE")")"
 
 source "$runDir/run-in-container.sh" "$testDir" "$1" sh -ec '
 	for c in pypy3 pypy python3 python; do
-		if PATH=/usr/local/bin command -v "$c" > /dev/null; then
-			exec "$c" "$@"
+		if [ -x "/usr/local/bin/$c" ]; then
+			exec "/usr/local/bin/$c" "$@"
 		fi
 	done
 	echo >&2 "error: unable to determine how to run python"