Include $PROJECT/bin in $PATH for gb generate

Projects may want to use binaries in $PROJECT/bin in `//go:generate`

Closes #630

Author: pritambaral

cmd/gb/generate.go

@@ -29,6 +29,7 @@ See 'go help generate'.
 	Run: func(ctx *gb.Context, args []string) error {
 		env := cmd.MergeEnv(os.Environ(), map[string]string{
 			"GOPATH": fmt.Sprintf("%s%c%s", ctx.Projectdir(), filepath.ListSeparator, filepath.Join(ctx.Projectdir(), "vendor")),
+			"PATH": fmt.Sprintf("%s%c%s", filepath.Join(ctx.Projectdir(), "bin"), filepath.ListSeparator, os.Environ["PATH"]),
 		})
 
 		cmd := exec.Command("go", append([]string{"generate"}, args...)...)