unix/linux: use HTTPS to git clone glibc in Dockerfile

This resolves a TODO to replace the insecure git protocol clone. There are no
changes to the generated code.

Change-Id: I66c83bcec572bf35a1206f8c82606cc122abb703
Reviewed-on: https://go-review.googlesource.com/c/sys/+/342553
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/unix/linux/Dockerfile b/unix/linux/Dockerfile
index 8f5bc27..da6b0a6 100644
--- a/unix/linux/Dockerfile
+++ b/unix/linux/Dockerfile
@@ -17,8 +17,8 @@
 RUN git config --global advice.detachedHead false
 # Linux Kernel: Released 27 Jun 2021
 RUN git clone --branch v5.13 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
-# GNU C library: Released 02 Aug 2021 (we should try to get a secure way to clone this)
-RUN git clone --branch release/2.34/master --depth 1 git://sourceware.org/git/glibc.git
+# GNU C library: Released 02 Aug 2021
+RUN git clone --branch release/2.34/master --depth 1 https://sourceware.org/git/glibc.git
 # Get Go