[tools] Fix local Go generator compilation on Windows

Fix a build failure on Windows inside the generate_sources step.

The generator runner script (`tools/run.py`) previously invoked `go run`
using an absolute path that begins with the drive letter (e.g.,
`C:\src\chromium...`). Go's package parser mistakes this for a remote
module name.

Furthermore, building within environments that inherit `GO111MODULE=off`
(like certain siso/ninja configurations) forces the compiler to run in
GOPATH mode. Under GOPATH mode, Go fails to locate local subpackages
within the `dawn.googlesource.com/dawn` module since they do not live
under `$GOPATH/src`.

This change addresses the issues by:
1. Passing a relative path starting with `./` and using forward slashes
   (e.g. `./tools/src/cmd/gen`) to `go run`, forcing the package path
   parser to resolve it as a directory.
2. Explicitly setting `GO111MODULE=on` inside the subprocess environment
   to force Go to compile the local project packages using module mode.

Bug: 519743871
Change-Id: I876afd94e06db8c4f424ddf9049e1e24a5c25f9a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/313675
Reviewed-by: Brian Sheedy <bsheedy@google.com>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Auto-Submit: Ganesh S. <ganesh@google.com>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>