std.Build.Step.Run: inherit build runner cwd

Right now, if you override the build root with `--build-root`, then
`Run` steps can fail to execute because of incorrect path handling in
the compiler: `std.process.Child` gets a cwd-relative path, but also has
its cwd set to the build root. The latter behavior is really weird; it
doesn't match my expectations, nor does it match how we spawn child
`zig` processes. So, this commit makes the child process inherit the
build runner's cwd, as `LazyPath.getPath2` *expects* it to.

After investigating, this behavior dates all the way back to 2017; it
was introduced in 4543413. So, there isn't any clear/documented reason
for this; it should be safe to revert, since under the modern `LazyPath`
system it is strictly a bug AFAICT.

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