Only load package environment file once when starting GHCi

Since d880d6b2 the parsing of the
environment files was moved to `parseDynamicFlags`, under the assumption
it was typically only called once. It turns out not to be true in GHCi
and this led to continually reparsing the environment file whenever a
new option was set, the options were appended to the package state and
hence all packages reloaded, as it looked like the options were changed.

The simplest fix seems to be a clearer specification:

> Package environment files are only loaded in GHCi during initialisation.

Fixes #19650