Skip extension testscript if bash isn't available

As this testscript creates a Bash-based script extension, the testscript should be skipped if it isn't on the path and executable.

Ideally, we would refactor this test to isolate that portion of the tests OR switch to a Go-based extension that can be compiled and run everywhere.

acceptance/testdata/extension/commandset.txtar

@@ -1,3 +1,6 @@
+# Skip if Bash is not available given script extension
+[!exec:bash] skip
+
 # Setup environment variables used for testscript
 env EXT_NAME=${SCRIPT_NAME}-${RANDOM_STRING}
 env EXT_SCRIPT=gh-${EXT_NAME}