build: make Borg.bin a static binary

This prevents it from e.g. needing a specific glibc version on the
client.

README.md

@@ -122,6 +122,8 @@ Notes
     pip install llfuse
     pip install -e .[llfuse]
     pyinstaller --clean --noconfirm scripts/borg.exe.spec
+    pip install staticx
+    staticx -l /lib/x86_64-linux-gnu/libm.so.6 dist/borg.exe Borg.bin
 
 Then see `dist/borg.exe`.  Confirm the version with `dist/borg.exe --version`.