Quote environ when injecting into crates vendor wrapper

[ryanpbrewster]

Slight tweak to the logic in #2862 (cc: @UebelAndre)

My $PATH variable has spaces in it and I hit an error like

env: Support/JetBrains/Toolbox/scripts:/Users/ryanpbrewster/.bin:/Users/ryanpbrewster/config/bin:/Users/ryanpbrewster/.local/bin:/Applications/Visual: No such file or directory                                                                     

If I manually modify bazel-bin/rust/crates_vendor.sh to modify

exec env - \
${_ENVIRON[@]} \

into

exec env - \
"${_ENVIRON[@]}" \

it starts working.

[illicitonion]

Thanks!