Cache GOROOT at init time

When GOROOT is not set as an environment variable, GetBuildContext
fetches it from 'go env GOROOT', which takes about 50ms on my machine.
This overhead can almost double the time it takes to run an update since
GetBuildContext is called relatively frequently.

Instead, read GOROOT during initialization and cache it for
GetBuildContext.

util/util.go

@@ -22,11 +22,25 @@ import (
 // other needs arise it may need to be re-written.
 var ResolveCurrent = false
 
+// goRoot caches the GOROOT variable for build contexts. If $GOROOT is not set in
+// the user's environment, then the context's root path is 'go env GOROOT'.
+var goRoot string
+
 func init() {
 	// Precompile the regular expressions used to check VCS locations.
 	for _, v := range vcsList {
 		v.regex = regexp.MustCompile(v.pattern)
 	}
+	if goRoot = os.Getenv("GOROOT"); len(goRoot) == 0 {
+		goExecutable := os.Getenv("GLIDE_GO_EXECUTABLE")
+		if len(goExecutable) <= 0 {
+			goExecutable = "go"
+		}
+		out, err := exec.Command(goExecutable, "env", "GOROOT").Output()
+		if err == nil {
+			goRoot = strings.TrimSpace(string(out))
+		}
+	}
 }
 
 func toSlash(v string) string {
@@ -272,6 +286,11 @@ func (b *BuildCtxt) PackageName(base string) string {
 //
 // TODO: This should be moved to the `dependency` package.
 func GetBuildContext() (*BuildCtxt, error) {
+	if len(goRoot) == 0 {
+		return nil, fmt.Errorf("Please set the $GOROOT environment " +
+				"variable to use this command\n")
+	}
+
 	buildContext := &BuildCtxt{build.Default}
 
 	// If we aren't resolving for the current system set to look at all
@@ -282,18 +301,7 @@ func GetBuildContext() (*BuildCtxt, error) {
 		buildContext.UseAllFiles = true
 	}
 
-	if goRoot := os.Getenv("GOROOT"); len(goRoot) == 0 {
-		goExecutable := os.Getenv("GLIDE_GO_EXECUTABLE")
-		if len(goExecutable) <= 0 {
-			goExecutable = "go"
-		}
-		out, err := exec.Command(goExecutable, "env", "GOROOT").Output()
-		if goRoot = strings.TrimSpace(string(out)); len(goRoot) == 0 || err != nil {
-			return nil, fmt.Errorf("Please set the $GOROOT environment " +
-				"variable to use this command\n")
-		}
-		buildContext.GOROOT = goRoot
-	}
+	buildContext.GOROOT = goRoot
 	return buildContext, nil
 }