Enhancement: use 1.18 build & source control info when available

[natefinch]

Describe the feature The only reason we have a build script for mage is to embed version information in the binary. As of 1.18, go will do that automatically. We should add code so that if you build with 1.18, we use that built-in info instead of the ldflags code that we use right now. That would allow anyone using 1.18 and above to simply go install github.com/magefile/mage@latest and not need to use the build script.

What problem does this feature address? It makes the install simpler and removes the need for the install script for anyone with a high enough version of go.