Avoid unfortunate IPC::Run path caching in PostgresNode

 Avoid unfortunate IPC::Run path caching in PostgresNode
 
 Commit b34ca595ab provided for installation-aware instances of
 PostgresNode. However, it turns out that IPC::Run works against this by
 caching the path to a binary and not consulting the path again, even if
 it has changed. We work around this by calling Postgres binaries with
 the installed path rather than just a bare name to be looked up in the
 environment path, if there is an installed path. For the common case
 where there is no installed path we continue to use the bare command
 name.
 
 Diagnosis and solution from Mark Dilger
 
 Discussion: https://postgr.es/m/E8F512F8-B4D6-4514-BA8D-2E671439DA92@enterprisedb.com