arch: fix file permissions for Windows pstore.
arch: fix file permissions for Windows pstore.

Without setting the share mode, we can't do occasionally useful things like
tail the pstore file.

BUG=b:237597358
TEST=builds + tested downstream on Windows

Change-Id: I893d0e52a671eeec5981527c1009b9d188110534
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3736759
Commit-Queue: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>
Reviewed-by: Vikram Auradkar <50abc1d85bedfd527fc7dee23f4819d1f0d946ec@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm