Run the pull request's natvis generator in the amalgamation check (#5873)

The check ran generate_natvis.py from the develop checkout and took only
the template from the pull request, so a pull request that changes the
generator (such as #5868, which limits the namespaces to two ABI tags)
could never pass: develop's generator rewrote the committed file. The
check now runs the pull request's generator, which loads its own
template, as the macro_builder step already does.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/check_amalgamation.yml

@@ -77,13 +77,12 @@ jobs:
           cd $MAIN_DIR
 
           # Built from this PR's own tools/macro_builder/main.cpp, not a
-          # develop checkout: unlike amalgamate.py and generate_natvis.py,
-          # this tool has no other source of truth to check against (its
-          # own README documents that it must reproduce macro_scope.hpp
-          # byte for byte), so there is nothing to gain from checking out a
-          # separate copy, and doing so would make this step fail on a PR
-          # that adds support for a new dispatch table until that PR itself
-          # merges to develop.
+          # develop checkout: unlike amalgamate.py, this tool has no other
+          # source of truth to check against (its own README documents that
+          # it must reproduce macro_scope.hpp byte for byte), so there is
+          # nothing to gain from checking out a separate copy, and doing so
+          # would make this step fail on a PR that adds support for a new
+          # dispatch table until that PR itself merges to develop.
           TMPDIR=$(mktemp -d ./macro_builder_check.XXXXXX)
           c++ -std=c++11 tools/macro_builder/main.cpp -o "$TMPDIR/macro_builder"
           "$TMPDIR/macro_builder" > "$TMPDIR/paste.hpp"
@@ -143,14 +142,14 @@ jobs:
       - name: Regenerate nlohmann_json.natvis
         run: |
           cd $MAIN_DIR
-          # The template is what this check verifies the committed natvis
-          # file against, so take it from this PR rather than from the
-          # develop checkout: generate_natvis.py loads the template from its
-          # own directory, and with develop's copy a PR that changes the
-          # template could never pass. Like the macro_builder step above,
-          # this only renders PR content in a job without credentials.
-          cp tools/generate_natvis/nlohmann_json.natvis.j2 $NATVIS_TOOL_DIR/
-          python3 $NATVIS_TOOL_DIR/generate_natvis.py $MAIN_DIR
+          # The generator and its template are what this check verifies the
+          # committed natvis file against (which namespaces it lists, and
+          # what each entry looks like), so run this PR's copy rather than
+          # the develop checkout's: with develop's, a PR that changes either
+          # could never pass. generate_natvis.py loads the template from its
+          # own directory. Like the macro_builder step above, this only runs
+          # PR content in a job without credentials.
+          python3 tools/generate_natvis/generate_natvis.py $MAIN_DIR
 
       - name: Build patch and check for differences
         id: diff