`spin build`: detect if invoking Spin, and if so use current Spin binary

[itowlson]

Although these are currently on the decrease with the new Python and JS SDKs, we have some cases in spin build where Spin invokes itself, specifically to run a plugin.

This is grand if Spin is on PATH, but is a problem if Spin is not on PATH, or the Spin on PATH is a different one from the one driving the build: this is biting the folks at Rancher Desktop (#2555).

We should consider detecting if Spin is invoking itself during build, and if so, to substitute in the path to the running Spin binary.

cc @jandubois