debian 10 ARMv7 job depends upon ld.lld which isn't installed in Docker image

Unfortunately, ARMv7 has a rather notable lack of functional linkers. ld.bfd suffers from binutils #16177 while ld.gold suffers from binutils #22266. Consequently, our only option is to depend instead upon ld.lld. I updated our CI configuration in 8635323b to do so but it seems that lld is not installed in our docker images. Fix this.

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