Workaround expired SSL certificate when building musl

As recommended in richfelker/musl-cross-make#151 (comment)

Current error is:

```
wget -c --no-verbose -O linux-headers-4.19.88-2.tar.xz https://ftp.barfooze.de/pub/sabotage/tarballs//linux-headers-4.19.88-2.tar.xz
 ERROR: cannot verify ftp.barfooze.de's certificate, issued by 'CN=R11,O=Let\'s Encrypt,C=US':
   Issued certificate has expired.
 To connect to ftp.barfooze.de insecurely, use `--no-check-certificate'.
```

Author: larskanis

build/mk_musl_cross.sh

@@ -29,7 +29,7 @@ GCC_CONFIG += --disable-libquadmath --disable-decimal-float
 GCC_CONFIG += --disable-multilib
 EOF
 
-make -j$(nproc) install
+make -j$(nproc) install LINUX_HEADERS_SITE=http://mirrors.2f30.org/sabotage/tarballs/
 
 popd