build/Makefile: Properly exclude files under "./vendor/" path for `go fmt` (#19097)

Properly exclude files under "./vendor/" path, and find "files" only.
The original method "might" found "directories" with the same naming,
or, might exclude occasionally exclude the go files with "vendor" string
in any part of its path.

Makefile

@@ -1,6 +1,6 @@
 VERSION?="0.3.32"
 TEST?=./...
-GOFMT_FILES?=$$(find . -name '*.go' | grep -v vendor)
+GOFMT_FILES?=$$(find . -not -path "./vendor/*" -type f -name '*.go')
 WEBSITE_REPO=github.com/hashicorp/terraform-website
 
 default: test