fix(cli): harden tht installer replacement

scripts/test-install-tht.sh

@@ -41,13 +41,15 @@ write_artifact "$artifact_one" "tht test version one"
 write_artifact "$artifact_two" "tht test version two"
 
 system_path=$PATH
+installer_bash=${THT_TEST_BASH:-/bin/bash}
+test -x "$installer_bash" || fail "THT_TEST_BASH must name an executable Bash interpreter"
 run_installer() {
   local artifact=$1
   THT_INSTALL_DIRECTORY="$test_directory" \
     THT_BUILD_ARTIFACT="$artifact" \
     THT_BUILD_ARTIFACT_SHA256="$(sha256 "$artifact")" \
     PATH="$test_directory:$system_path" \
-    bash "$installer"
+    "$installer_bash" "$installer"
 }
 
 run_installer "$artifact_one"