Rollup merge of #45343 - bgermann:master, r=alexcrichton
Rollup merge of #45343 - bgermann:master, r=alexcrichton

Make Solaris builder compatible with Solaris 10 retry

Unfortunately, #45255 does not quite cut it,
so use a different approach to have Solaris 10 compatibility
by tricking libbacktrace's autoconf tests.
The sysroot download routine is slightly changed, too.

GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust