ci/docs: fix base-href in matrix

For some reason, when updating `base-href` using `+=`, yq updates _all_
entries matching the LHS selection. This means that all non-main
branches get `"base-href": "/nixvim/24.11/"` in the job matrix.

This gets passed in as an overridden attr when building the docs, and is
used to determine which list item represents the currently-being-built
docs; therefore both 25.05 and 24.11 think they are 24.11...

For some reason, while `+=` does this, `=` does not. So switched to
using that.

(cherry picked from commit b4750c4696)

.github/workflows/website.yml

@@ -45,7 +45,7 @@ jobs:
                     "base-href": "/nixvim/"
                   }
                 | select(.ref != "main").sub-path = .name
-                | select(.ref != "main").base-href += .name + "/"
+                | select(.ref != "main").base-href = "\(.base-href)\(.name)/"
               ]
             ' version-info.toml