The search_path is set per session and can be changed any time. You can store presets for a database, a role or even a role in a particular database. That would be set with commands like:
ALTER DATABASE test SET search_path = blarg,public;
ALTER ROLE foo SET search_path = blarg,public;
Etc.
Or maybe you want a general setting in postgresql.conf to begin with? See link below.
A plain:
SET search_path = blarg,public;
like you see in SQL file only sets a search_path for the session it is executed in.
The underlying issue may be this (quoting the manual):
Database roles are global across a database cluster installation (and
not per individual database).
Bold emphasis mine.
A backup of a database with pg_dump (not the whole cluster with pg_dumpall, not including global objects), does not include roles. If you have set a default search_path for a role, then that is not included.
Details: