fix(cargo_build_script): expand ${pwd} for execpaths and locations in build_script_env (#4088)

Singular `$(execpath ...)` and `$(location ...)` were already prefixed
with `${pwd}/` so build_script_runner could resolve them to absolute
paths at run time. The plural forms `$(execpaths ...)` and `$(locations
...)` were silently omitted from this handling, leaving their paths as
execroot-relative.

For plural forms each macro is expanded individually (via
ctx.expand_location) and every resulting space-separated path is
prefixed with `${pwd}/`.