Freeze environment reading after saving deps

Any reads after the deps are saved won't show up as dependencies later.
So panic if new environment variables are read after saving deps.

Change-Id: Ia51deaf750804d3b99e69c001939a104c2d8c9f2
diff --git a/common/config.go b/common/config.go
index 348d0db..f8231f0 100644
--- a/common/config.go
+++ b/common/config.go
@@ -48,8 +48,9 @@
 
 	srcDir string // the path of the root source directory
 
-	envLock sync.Mutex
-	envDeps map[string]string
+	envLock   sync.Mutex
+	envDeps   map[string]string
+	envFrozen bool
 }
 
 type jsonConfigurable interface {
@@ -178,6 +179,9 @@
 	var exists bool
 	c.envLock.Lock()
 	if val, exists = c.envDeps[key]; !exists {
+		if c.envFrozen {
+			panic("Cannot access new environment variables after envdeps are frozen")
+		}
 		val = os.Getenv(key)
 		c.envDeps[key] = val
 	}
@@ -186,6 +190,9 @@
 }
 
 func (c *config) EnvDeps() map[string]string {
+	c.envLock.Lock()
+	c.envFrozen = true
+	c.envLock.Unlock()
 	return c.envDeps
 }