Tested on Linux, Mac and Windows with space in Arduino IDE folder path, Sketch path and build output path. Tested also with VSCode. PIO is not impacted as the prebuild is not executed.

I recommend to put the script in system. it is an 'official' folder mapped to {build.system.path}.
'extras' folder could make problems for hardware definitions referencing this package.
I don't know yet if this core works as referenced according to Arduino hw package specs, but the 'extras' folder would definitely not work.

Thansk @JAndrassy
That make sense.

@JAndrassy

Refering to https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification:

Global Predefined properties

The Arduino IDE sets the following properties that can be used globally in all configurations files:

{runtime.platform.path}     - the absolute path of the platform folder (i.e. the folder containing boards.txt)

and:

Moreover the IDE automatically provides the following properties:

{build.system.path}       - The path to the selected platform's system folder if available (for example hardware/arduino/sam/system)

So I wonder if this will change anything as {runtime.platform.path} is also an 'official' folder.

so updating to:

- extras.path={runtime.platform.path}/extras
+ extras.path={build.system.path}/extras

will not change the behavior.

{runtime.platform.path}, points to the folder of selected board's boards.txt file. In case of a referring definition it is not the right folder to access sdk and tools. {build.system.path} is right for this.

a core can be referred by other core or by a custom hardware definition.
here I have my customized definitions for avr and samd. they refer the official avr and samd cores.
https://github.com/jandrassy/my_boards

EDIT 2020-03-05: I added BluePill to my_boards

OK. I don't think about this use case.
Thanks for pointing out this.

{runtime.platform.path}, points to the folder of selected board's boards.txt file. In case of a referring definition it is not the right folder to access sdk and tools. {build.system.path} is right for this.

See arduino-cli code (used by current arduino-builder) for how this is implemented.

I've also updated https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification to clarify how this works.