Set XBUILD_SYSROOT_PATH when building bootloader

[phil-opp]

The XBUILD_SYSROOT_PATH environment variable controls where cargo-xbuild should create the custom sysroot. This PR sets the variable to target/bootloader-sysroot when building the bootloader.

The previous behavior was to create the sysroot in a target/sysroot directory under the bootloader source directory, which is normally a folder in the local cargo package registry (under ~/.cargo). Modifying the content of this folder does not seem like a good idea. It also caused problems when the .cargo folder lives under a path that contains spaces due to rust-lang/cargo#6139.

This PR also unset the SYSROOT_DIR environment variable which seems unused.

A side effect of this change is that it should fix the CI build. It was broken for a few days because Azure Pipelines now includes rustup in its Windows image, so that the .cargo folder lives under Program Files, which leads to a sysroot path with spaces when building the bootloader without the XBUILD_SYSROOT_PATH variable.

[phil-opp]

Released as version 0.7.5