Use trimpath for compatibility with go-ipfs v0.7.0

[aschmahmann]

This PR changes the build flags used when building plugins.

There are 3 types of go-ipfs we support building plugins against:

1. The release binaries distributed at dist.ipfs.io
2. A locally built go-ipfs
3. A Git referred version (e.g. commit/branch/tag) of go-ipfs

Previously:
Use the -asmflags=all=-trimpath="$(GOPATH)" -gcflags=all=-trimpath="$(GOPATH)" flags no matter which version of the plugin we're building against.

This was a pain when building against a Git commit/branch/tagged version of go-ipfs since the complex set of flags above had to be passed into go get github.com/ipfs/go-ipfs/cmd/ipfs@{commit|branch|tag}. Additionally, the use of GOPATH here may not be correct depending on how the users' directory structure is set up.

Now:
Use the -asmflags=all=-trimpath="$(GOPATH)" -gcflags=all=-trimpath="$(GOPATH)" only when building against a locally build copy of go-ipfs, otherwise use the -trimpath flag instead.

This means we can run go get -trimpath github.com/ipfs/go-ipfs/cmd/ipfs@{commit|branch|tag} and just build the plugin with -trimpath (as done in the Makefile here) and things will work.

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[aschmahmann]

@Stebalien updated, take a look when you get a chance 🙏