feat(opentf): Update opentofu to current release

images/opentf/.release

-release=1.6.0-alpha2
+release=1.6.0

images/opentf/Earthfile

@@ -2,7 +2,8 @@ VERSION 0.7
 
 src:
     FROM ../mirror+golang
-    ARG branch=main
+    COPY .release ./
+    ARG branch=v$(awk -F'=' '$1 == "release" {print $2}' .release)
     WORKDIR /go-workdir
     GIT CLONE --branch=${branch} https://github.com/opentofu/opentofu.git ./
     SAVE ARTIFACT ./ ./src