Avoid redundant downloads when bootstrapping

If the local file is available, then verify it against the hash we just
downloaded, and if it matches then we don't need to download it again.

src/bootstrap/bootstrap.py

@@ -31,6 +31,14 @@ def get(url, path, verbose=False):
 
     try:
         download(sha_path, sha_url, verbose)
+        if os.path.exists(path):
+            try:
+                verify(path, sha_path, verbose)
+                print("using already-download file " + path)
+                return
+            except Exception as e:
+                print("failed verification for already-download file " + path)
+                os.unlink(path)
         download(temp_path, url, verbose)
         verify(temp_path, sha_path, verbose)
         print("moving {} to {}".format(temp_path, path))

src/etc/get-stage0.py

@@ -31,8 +31,6 @@ def main(triple):
     filename = 'rustc-{}-{}.tar.gz'.format(channel, triple)
     url = 'https://static.rust-lang.org/dist/{}/{}'.format(date, filename)
     dst = dl_dir + '/' + filename
-    if os.path.exists(dst):
-        os.unlink(dst)
     bootstrap.get(url, dst)
 
     stage0_dst = triple + '/stage0'