Make rust builds hermetic

Rust builds were picking up the crt objects and system libraries like
libc.so from /usr/lib/x86_64-linux-gnu/Scrt1.o and
/lib/x86_64-linux-gnu/libc.so.6.  Pass --sysroot to the linker to
point it to the glibc prebuilts.

Test: manual
Change-Id: I6540df8aef0e5c2258de77456d18a6052db627d4