SetEnvPrefix was broken: it set the prefix string but never re-read
environment variables. This meant collapse() and SetDefault() couldn't
match prefixed env vars to their unprefixed config keys, so defaults
always won over env vars.
The fix makes SetEnvPrefix behave identically to WithEnvPrefix: it
re-reads os.Environ(), strips the prefix from matching keys, and
stores the stripped keys in envConfig. The envPrefix field is removed
entirely since keys are always pre-stripped.
BREAKING CHANGE: SetEnvPrefix now filters env vars to only those
matching the prefix (previously all env vars were accessible).
This matches the documented and expected behavior.
