Fix issues with 'make generate'

[jhump]

I happened to notice errors on my local machine when I would run make generate. I did a little digging and realize these were material errors that were happening in CI, too.

If you look at this latest CI job from main, in the "Unit Test" step, you'll see output that repeats the following error message:

bash: line 1: buf: command not found

On my laptop, I was seeing a different error:

Failure: plugin protoc-gen-connect-go: exec: "protoc-gen-connect-go": executable file not found in $PATH

Doh! This error should have caused the make generate target to halt and fail, but it did not. This pull request remedies that.

There are two different issues afoot:

1. The generate.sh script would return success (zero exit code), even when the various commands it was running would fail. This is due to how the -exec flag to find works. So I changed it to instead use xargs, which will properly return a non-zero exit code if any of the processes it spawns fails.
2. The next issue was the underlying cause for the commands in generate.sh failing: the PATH variable was not properly configured. So the make generate target was building the dependencies but not making them available on the PATH.

Now that this is fixed, this also fixes the generated code produced by that step so that it corresponds to the correct version of protoc-gen-go (i.e. the version pinned in go.mod).