Lock picotool version on master to 2.1.1

The master SDK branch (and the SDK release tags eg 2.1.1) should point to specific picotool tags, to ensure you get a compatible picotool
diff --git a/tools/Findpicotool.cmake b/tools/Findpicotool.cmake
index e5bfaea..4720a79 100644
--- a/tools/Findpicotool.cmake
+++ b/tools/Findpicotool.cmake
@@ -37,7 +37,7 @@
         message("Downloading Picotool")
         FetchContent_Populate(picotool QUIET
             GIT_REPOSITORY https://github.com/raspberrypi/picotool.git
-            GIT_TAG develop
+            GIT_TAG 2.1.1
 
             SOURCE_DIR ${picotool_INSTALL_DIR}/picotool-src
             BINARY_DIR ${picotool_INSTALL_DIR}/picotool-build