Auto merge of #36456 - alexcrichton:rustbuild-dont-use-c, r=jonathandturner
Auto merge of #36456 - alexcrichton:rustbuild-dont-use-c, r=jonathandturner

rustbuild: Use current_dir instead of -C

Apparently some versions of git don't support the `-C` flag, so let's use the
guaranteed-to-work `current_dir` function.

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