Add 'clang' back to the matrix in '.github/workflows/main.yml'

src/tests/script.regexps.vtc

@@ -29,7 +29,11 @@ EOF
 ## Test: regmatch().
 ##
 
-varnish v1 -vcl+backend {
+# When using '--disable-luajit' to compile the VMOD, a segmentation fault will
+# ocurr if not increasing the 'thread_pool_stack' parameter (see
+# https://github.com/varnishcache/varnish-cache/issues/2907). A different way
+# to avoid the problem is using '--disable-pcre-jit' when compiling Varnish.
+varnish v1 -arg "-p thread_pool_stack=128k"  -vcl+backend {
     import cfg from "${vmod_topbuild}/src/.libs/libvmod_cfg.so";
 
     sub vcl_init {
@@ -159,7 +163,7 @@ varnish v1 -expect MGT.child_panic == 0
 # ocurr if not increasing the 'thread_pool_stack' parameter (see
 # https://github.com/varnishcache/varnish-cache/issues/2907). A different way
 # to avoid the problem is using '--disable-pcre-jit' when compiling Varnish.
-varnish v2 -arg "-p thread_pool_stack=256k" -vcl+backend {
+varnish v2 -arg "-p thread_pool_stack=128k" -vcl+backend {
     import cfg from "${vmod_topbuild}/src/.libs/libvmod_cfg.so";
 
     sub vcl_init {