Tools overridden via --prefs are persistent

[facchinm]

The bug gets exposed in a very particular configuration (and is usually harmless).

Steps to reproduce:
1 - Select a board with a board manager entry (MKRWIFI1010 for example)
2 - Compile a sketch
3 - Change board with one contained in the sketchbook (one from the mbed core with openocd as programmer)
4 - Select "upload using programmer"
5 - The wrong tool version will be selected

If steps 1 and 2 are not performed the right version gets selected.
If between step 2 and 3 the user opens the board manager the correct version gets selected too.

Caused by:
These lines

Arduino/arduino-core/src/cc/arduino/Compiler.java

Lines 269 to 273 in c4109e7

	PreferencesData.getMap()
	.subTree("runtime.build_properties_custom")
	.entrySet()
	.stream()
	.forEach(kv -> cmd.add("-prefs=" + kv.getKey() + "=" + kv.getValue()));

allow overriding the automatically discovered tools, but should be cleared when we change the target board (and they are not)