Do not allow update_file.py to be called on output.txt (#2847)

## Tests
Manually:
```
➜  cli git:(do-not-allow-output) ./acceptance/bin/update_file.py output.txt a b
Traceback (most recent call last):
  File "/Users/shreyas.goenka/repos/cli/./acceptance/bin/update_file.py", line 21, in <module>
    assert filename != "output.txt"
AssertionError
```

Author: shreyas-goenka

acceptance/bin/update_file.py

@@ -10,6 +10,16 @@
 import sys
 
 filename, old, new = sys.argv[1:]
+
+# Acceptance tests by default keep output.txt open to append output to.
+# Using update_file.py on output.txt can be flaky on windows, likely because
+# of some internal buffering with the file handle. Thus we do not allow updating
+# output.txt with this script.
+#
+# You are instead recommended to write the output to a different file and
+# call update_file.py on that file.
+assert filename != "output.txt"
+
 data = open(filename).read()
 newdata = data.replace(old, new)
 if newdata == data: