This should lead to a small performance gain by caching the openapi.yml with etags as well as eliminating the use of nonce crypto generation when not required.
This commit replaces `os.Setenv` with `t.Setenv` in tests. The environment variable is automatically restored to its original value when the test and all its subtests complete. Reference: https://pkg.go.dev/testing#T.Setenv
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>