chore: run macOS, windows, and race tests with Postgres in CI (#15520)

This PR is the second in a series aimed at closing
https://github.com/coder/coder/issues/15109.

## Changes

- adds `scripts/embedded-pg/main.go`, which can start a native Postgres
database. This is used to set up PG on Windows and macOS, as these
platforms don't support Docker in Github Actions.
- runs the `test-go-pg` job on macOS and Windows too
- adds the `test-go-race-go` job, which runs race tests with Postgres on
Linux

.github/actions/setup-imdisk/action.yaml

@@ -0,0 +1,27 @@
+name: "Setup ImDisk"
+if: runner.os == 'Windows'
+description: |
+  Sets up the ImDisk toolkit for Windows and creates a RAM disk on drive R:.
+runs:
+  using: "composite"
+  steps:
+    - name: Download ImDisk
+      if: runner.os == 'Windows'
+      shell: bash
+      run: |
+        mkdir imdisk
+        cd imdisk
+        curl -L -o files.cab https://github.com/coder/imdisk-artifacts/raw/92a17839ebc0ee3e69be019f66b3e9b5d2de4482/files.cab
+        curl -L -o install.bat https://github.com/coder/imdisk-artifacts/raw/92a17839ebc0ee3e69be019f66b3e9b5d2de4482/install.bat
+        cd ..
+
+    - name: Install ImDisk
+      shell: cmd
+      run: |
+        cd imdisk
+        install.bat /silent
+
+    - name: Create RAM Disk
+      shell: cmd
+      run: |
+        imdisk -a -s 4096M -m R: -p "/fs:ntfs /q /y"