Build process should start in the sketch directory

[AlexIII]

Description

Building with arduino-builder.exe should start in the sketch directory in order to comply to Arduino IDE.
Right now it starts from the extension's some sub-directory.

This leads to problems when including files with relative path from inline assembler, because relative path in inline assembler is relative to the current directory in which the compiler is running (and not to the source file).

For example, it makes it impossible to use incbin which works fine in the original Arduino IDE.

Test case

TEST1/test.bin - empty file

TEST1/sketch1/sketch1.ino

__asm__ (
    ".incbin \"../test.bin\"\n"
);
void setup() {}
void loop() {}

This sketch will compile with Arduino IDE, but not with vscode-arduino.

[AlexIII]

PR provided

[benmcmorran]

PR merged.