build(cargo): set -Ctarget-feature=+crt-static

I tried running prebuilt binaries from GitHub Actions on a fresh Windows
11 VM and was faced with the error "The code execution cannot proceed
because VCRUNTIME140.dll was not found."

After Googling around a little I think this may help with that issue;
definitely don't want to have to troubleshoot this for non-developers
who just want to use a tiling window manager.

.github/workflows/windows.yaml

@@ -20,6 +20,8 @@ jobs:
   build:
     name: Build
     runs-on: windows-latest
+    env:
+      RUSTFLAGS: -Ctarget-feature=+crt-static
     strategy:
       fail-fast: false
       matrix: