Re-apply temporary workaround to generateThemeSite.sh

@digitalcraftsman was trying to re-apply this workaround
in commit 3068f64,
but for some reason that commit contains whitespace cleanup only.
The following commit message was from that commit:

Setting the theme path via the `-t THEME` flag isn't recognized
and still requires to use the HUGO_THEME env variable as workaround.

See #412
See gohugoio/hugo#5061
See gohugoio/hugo#4851

This reverts commit 0b4e98b.

_script/generateThemeSite.sh

@@ -182,7 +182,7 @@ for x in `find ${themesDir} -mindepth 1 -maxdepth 1 -type d -not -path "*.git" -
             ln -s ${themesDir}/$x/exampleSite ${siteDir}/exampleSite2
             ln -s ${themesDir} ${siteDir}/exampleSite2/themes
             destionation="../themeSite/static/theme/$x/"
-            hugo --quiet -s exampleSite2 -d ${demoDestination} -t $x -b $BASEURL/theme/$x/
+            HUGO_THEME=${x} hugo --quiet -s exampleSite2 -d ${demoDestination} -b $BASEURL/theme/$x/
             if [ $? -ne 0 ]; then
                 echo "FAILED to create exampleSite for $x"
                 errorCounter=$((errorCounter + 1))
@@ -210,7 +210,7 @@ for x in `find ${themesDir} -mindepth 1 -maxdepth 1 -type d -not -path "*.git" -
             cat themeSite/templates/${paramsConfig} >>${themeConfig}
 
             echo "Building site for theme ${x} using config \"${themeConfig}\" to ${demoDestination}"
-            hugo --quiet -s exampleSite --config=${themeConfig} -d ${demoDestination} -t $x -b $BASEURL/theme/$x/
+            HUGO_THEME=${x} hugo --quiet -s exampleSite --config=${themeConfig} -d ${demoDestination} -b $BASEURL/theme/$x/
             if [ $? -ne 0 ]; then
                 echo "FAILED to create demo site for $x"
                 rm -rf ${demoDestination}