Fix empty $pgvm_home env variable

After the commit 76bbde624f8434b4bc3d28e037dd99e4cc99f464 the pgvm command fails with "pgvm: command not found" message on my ubuntu.

I think this happens because we are escaping the $pgvm_home variable when defining it [1], so I requesting this pull to fix it.

[1] https://github.com/guedes/pgvm/commit/76bbde624f8434b4bc3d28e037dd99e4cc99f464#L4R45