Replace #!/bin/bash with #!/usr/bin/env bash to improve compatibility with systems where /bin/bash is not available

tests/syntax-tests/regression_test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -eou pipefail
 

tests/syntax-tests/update.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 python="python3"
 if ! command -v python3 &>/dev/null; then python="python"; fi