chore: add test_instrumented_firecracker_build to gitignore

The folder is created when running integration tests,
and doesn't belong in git.

Signed-off-by: Marco Marangoni <mamarang@amazon.com>

Author: marco-marangoni

.gitignore

@@ -18,3 +18,4 @@ tags
 /resources/x86_64
 /resources/aarch64
 redundant_seccomp_rules_build
+test_instrumented_firecracker_build