emsdk

fix: improve Fish shell support (and Cshell) (#1429)

I faced the same issue mentioned in #1299, this PR fixes the issue by
taking in account specific `set` and `unset` syntax for Fish shell.

The post install instruction (i.e : `./emsdk activate latest`) provides
wrong instructions in Fish shell and Cshell cases.
This should be fixed by this PR too, although the code is a bit
redundant and it could be more cleaner.