tests: fbs test: Fix issues around missing git

- update_fbs.py would use git directly, instead of the requested git
  executable
- There are other corner cases for the git executable detection, which
  are now all captured under a more general exception type

Credit to Christopher Friedt for pointing out the original issue.

host/tests/verify_fbs_test.py

@@ -26,7 +26,8 @@ class VerifyFBSTest(unittest.TestCase):
         spec.loader.exec_module(update_fbs)
         try:
             git_exe = update_fbs.find_executable("git")
-        except RuntimeError:
+        # pylint: disable=broad-except
+        except Exception:
             # No git, no test. We pass b/c git is not a UHD dependency.
             return
         sys.argv.append('--verify')

host/utils/update_fbs.py

@@ -108,9 +108,12 @@ def verify(git_exe, uhd_path=None):
     accompanied by a change of the header. It also detects manual
     changes to the generated header files.
     """
+    if not git_exe:
+        print("Cannot verify schema files (no git found), assuming pass")
+        return True
     try:
-        subprocess.check_output(("git", "status"), stderr=subprocess.STDOUT)
-    except subprocess.CalledProcessError as error:
+        subprocess.check_output((git_exe, "status"), stderr=subprocess.STDOUT)
+    except subprocess.CalledProcessError:
         print("Cannot verify schema files (not a git repo), assuming pass")
         return True
     try: