better support for the "pendrive" use case

[mvdan]

That is, a use case where someone runs IPFS from a pendrive, without installing it on the system.

Right now we look for ipfs in $PATH. We should also look for it somewhere local; for example, right next to where the go-ipfs-desktop binary is.

[lidel]

I'd go with lookup order:

• the same directory as go-ipfs-desktop binary (realpath go-ipfs-desktop)
  • this will often be different from the current directory (pwd)
• $PATH