docs/ci: use nix build --argstr to safely supply input

Rather than interpolating input (flakeref & baseHref) into the build
expression, take advantage of `--argstr`.

See "Common evaluation options" in `nix build --help` for more detail.

.github/workflows/build_documentation.yml

@@ -53,15 +53,30 @@ jobs:
           # 1: branch
           # 2: install dir (relative to /nixvim/)
           build() {
-            flake="github:${repo}${1:+/$1}"
+            branch="$1"
             dir="${2:+/$2}"
+            flakeref="github:${repo}${branch:+/$branch}"
             baseHref="/${repoName}${dir}/"
             installDir="${out}${dir}"
 
             # Build docs for the given flake ref, overriding baseHref in the derivation args
-            nix build --impure --expr '(builtins.getFlake "'"$flake"'").outputs.packages.${builtins.currentSystem}.docs.override {
+            nix build --impure \
-              baseHref = "'"$baseHref"'";
+                --argstr flakeref "$flakeref" \
-            }'
+                --argstr baseHref "$baseHref" \
+                --expr '
+                  {
+                    flakeref,
+                    baseHref,
+                    system ? builtins.currentSystem,
+                  }:
+                  let
+                    flake = builtins.getFlake flakeref;
+                    packages = flake.outputs.packages.${system};
+                  in
+                  packages.docs.override {
+                    inherit baseHref;
+                  }
+                '
 
             # Copy the result to the install dir
             mkdir -p "$installDir"