Move top-level installer directory to ci (#527)

A small amount of cleanup for the top-level of this repository, where
the `installer` directory just had one misc file for the Windows
installer which we can put elsewhere inside of `ci`, another misc folder
but one that already has a few files in it.

.github/workflows/main.yml

@@ -249,7 +249,7 @@ jobs:
     # Make a Windows MSI installer if we're on Windows
     - run: |
         export WT_VERSION=`cat Cargo.toml | sed -n 's/^version = "\([^"]*\)".*/\1/p'`
-        "$WIX/bin/candle" -arch x64 -out target/wasmtime.wixobj installer/msi/wasmtime.wxs
+        "$WIX/bin/candle" -arch x64 -out target/wasmtime.wixobj ci/wasmtime.wxs
         "$WIX/bin/light" -out dist/installer.msi target/wasmtime.wixobj -ext WixUtilExtension
         rm dist/installer.wixpdb
       shell: bash