Make check-flagged-apis executable

Occasionally sourcing the script would close my overall shell when it
hit an error. By having it just be an executable script, this prevents
it from impacting a user's normal shell environment and can depend on it
always being bash, rather than whatever shell people happen to be using.

Bug: 334870672
Test: tools/check-flagged-apis/check-flagged-apis.sh
Change-Id: Ic46cb4fefdea8d51be018d4f7a92b0d9ca7e57b3