cmd/coordinator: set GO_BUILDER_NAME env var when exec'ing on buildlet

This will allow tests to identify that they are running on the
continuous build infrastructure and run extra and/or different tests.

Fixes golang/go#11346

Change-Id: Iafe25babc764b7e012c05f0b62331b11690bf194
Reviewed-on: https://go-review.googlesource.com/13120
Reviewed-by: Russ Cox <[email protected]>

Author: adg

dashboard/builders.go

@@ -55,7 +55,9 @@ type BuildConfig struct {
 	allScriptArgs []string
 }
 
-func (c *BuildConfig) Env() []string { return append([]string(nil), c.env...) }
+func (c *BuildConfig) Env() []string {
+	return append([]string("GO_BUILDER_NAME="+c.Name), c.env...)
+}
 
 func (c *BuildConfig) GOOS() string { return c.Name[:strings.Index(c.Name, "-")] }