Pin Go to 1.20.5 to work around testcontainers/Go breakage (#40)

Pin Go to version `1.20.5` to fix
[testcontainer](https://golang.testcontainers.org/) tests.

Go `1.20.6` introduced changes in `Host` header parsing
([issue](golang/go#61431)) which breaks
`testcontainers-go`
([issue](testcontainers/testcontainers-go#1359)).

Our tests started failing consistently yesterday due to this issue
([example
run](https://github.com/xataio/pg-roll/actions/runs/5612153506)).

The suggested workaround until this is resolved upstream is to pin to
`1.20.5`.

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: '1.20'
+        go-version: '1.20.5'
 
     - name: Run tests
       run: go test ./...
@@ -24,7 +24,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: '1.20'
+        go-version: '1.20.5'
         cache: false
 
     - name: golangci-lint