[docs] Call qhelpgenerator from Qt6.

Author: hageboeck

.github/workflows/root-docs-ci.yml

@@ -72,8 +72,8 @@ jobs:
     - name: Prepare qhelpgenerator
       # Qt6's libexec executables are not in the PATH by default
       run: |
-        which qhelpgenerator && exit 0 # Already in PATH
-        PATH=/usr/lib64/qt6/libexec/:${PATH} which qhelpgenerator && echo "/usr/lib64/qt6/libexec/" >> "${GITHUB_PATH}"
+        QHELPGENERATOR=$(which qhelpgenerator) && echo "QHELPGENERATOR=${QHELPGENERATOR}" >> "${GITHUB_ENV}"
+        PATH=/usr/lib64/qt6/libexec/:${PATH} QHELPGENERATOR=$(which qhelpgenerator) && echo "QHELPGENERATOR=${QHELPGENERATOR}" >> "${GITHUB_ENV}"
 
     - name: Checkout
       uses: actions/checkout@v6

documentation/doxygen/Doxyfile

@@ -1700,7 +1700,7 @@ QHP_SECT_FILTER_ATTRS  =
 # run qhelpgenerator on the generated .qhp file.
 # This tag requires that the tag GENERATE_QHP is set to YES.
 
-QHG_LOCATION           = qhelpgenerator
+QHG_LOCATION           = $(QHELPGENERATOR)
 
 # If the GENERATE_ECLIPSEHELP tag is set to YES, additional index files will be
 # generated, together with the HTML files, they form an Eclipse help plugin. To