demo: fix minor build issues

[tpwrules]

Fixes a couple minor issues with the bare metal software demo encountered under Nix.

1. Programs are stored without write permissions. The demo data copied to the build directory is made writable after the copy so the linker script can be modified and it can be built.

2. The Python executable is a wrapper which also sets up library paths and so forth. The current Python executable is reused to generate the final output binary instead of whichever Python python3 is to ensure the version of LiteX used is consistent.