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>
Avoid unnecessary byte/string conversion
We can use alternative functions/methods to avoid unnecessary
byte/string conversion calls.
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>