Lock picotool version on master to 2.1.1 (hathach#2401)

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 (note this is beyond the 2.1.1 tag in master, so will just fix this for users who check out the HEAD of master)

Author: will-v-pi

tools/Findpicotool.cmake

@@ -37,7 +37,7 @@ if (NOT TARGET picotool)
         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