ci: Don't use GNU iconv for legacy builds

We are only using GNU iconv instead of native one because in newer macOS
versions the system iconv is buggy. This isn't an issue for older macOS
versions. Building with GNU iconv makes the build chain more complicated
for self-hosted VMs and it also increases binary size so we don't want
to do this unless it's necessary.

.github/workflows/macvim-buildtest.yaml

@@ -89,10 +89,10 @@ jobs:
           xcode-select -p
           xcodebuild -version
 
-      # Set up, install, and cache GNU libiconv library to work around Apple iconv issues.
+      # Set up, install, and cache GNU libiconv library to work around Apple iconv issues in newer macOS versions.
 
       - name: Set up libiconv
-        if: inputs.publish
+        if: inputs.publish && !inputs.legacy
         uses: ./.github/actions/universal-package
         with:
           formula: libiconv
@@ -106,7 +106,7 @@ jobs:
         with:
           formula: gettext
           contents: lib/libintl.a,lib/libintl.dylib
-          gnuiconv: true  # gettext needs to match MacVim in using the same version of iconv
+          gnuiconv: ${{ inputs.legacy == false }}  # gettext needs to match MacVim in using the same version of iconv
 
       # Set up, install, and cache libsodium library for encryption.
 
@@ -232,7 +232,7 @@ jobs:
             sed -i.bak -f ci/config.mk.optimized.sed src/auto/config.mk
           fi
 
-          if ${{ inputs.publish == true }}; then
+          if ${{ inputs.publish == true && inputs.legacy == false }}; then
             # Use Homebrew GNU libiconv since Apple iconv has been broken since macOS 14
             sed -i.bak -f ci/config.mk.brew-libiconv.sed src/auto/config.mk
           fi
@@ -286,7 +286,7 @@ jobs:
             echo 'Found external dynamic linkage!'; false
           fi
 
-          if ${{ inputs.publish == true }}; then
+          if ${{ inputs.publish == true && inputs.legacy == false }}; then
             # Make sure we are not using system iconv, which has been buggy since macOS 14.
             if otool -L ${VIM_BIN} | grep '^\s*/usr/lib/libiconv'; then
               echo 'Using system iconv! We should be linking against GNU iconv instead.'; false