Use github.com Linux kernel mirror in integration test

CI jobs running on github are less likely to temporarily fail when using
mirrors from github.com. The (transient?) failure seen most recently
was:
```
git clone -b v5.10 --depth=1 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git linux_5_10
Cloning into 'linux_5_10'...
remote: aborting due to possible repository corruption on the remote side.
fatal: early EOF
fatal: fetch-pack: invalid index-pack output
```
This commit seeks to address this kind of problem.

Author: tautschnig

integration/linux/compile_linux.sh

@@ -21,7 +21,7 @@ make -C src CXX='ccache /usr/bin/g++' cbmc.dir goto-cc.dir goto-diff.dir -j$(npr
 [ -d one-line-scan ] || git clone https://github.com/awslabs/one-line-scan.git one-line-scan
 
 # Get Linux v5.10, if we do not have it already
-[ -d linux_5_10 ] || git clone -b v5.10 --depth=1 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git linux_5_10
+[ -d linux_5_10 ] || git clone -b v5.10 --depth=1 https://github.com/torvalds/linux/ linux_5_10
 
 # Prepare compile a part of the kernel with CBMC via one-line-scan
 ln -s goto-cc src/goto-cc/goto-ld