[ci,cs] workaround random output mixup for csTwoLibs test

CI sometimes (3 times over 30 runs when I measured) ends up mixing up
dll output of this test, and tries to compile (natively) code compiled
both with and without -D no-root

Clearing output folder between runs seems to make the problem disappear

tests/misc/cs/csTwoLibs/compile-1.hxml

@@ -1,3 +1,5 @@
+--cmd rm -rf bin
+--next
 cs.Boot
 --dce no
 -cs bin/haxeboot

tests/misc/cs/csTwoLibs/compile-2.hxml

@@ -1,3 +1,5 @@
+--cmd rm -rf bin
+--next
 cs.Boot
 --dce no
 -cs bin/haxeboot

tests/misc/cs/csTwoLibs/compile-3.hxml

@@ -1,3 +1,5 @@
+--cmd rm -rf bin
+--next
 cs.Boot
 --dce no
 -cs bin/haxeboot

tests/misc/cs/csTwoLibs/compile-4.hxml

@@ -1,3 +1,5 @@
+--cmd rm -rf bin
+--next
 cs.Boot
 --dce no
 -cs bin/haxeboot