xtask: release mode

Since a7cfc694bd we are unpacking dpkg
archives in native Rust and since ... we are downloading and unpacking
debug archives which are around 10x larger than kernel archives for the
same version. Thus build with optimizations so we spend less time in
archive extraction.

.cargo/config.toml

@@ -1,5 +1,5 @@
 [alias]
-xtask = "run --package xtask --"
+xtask = "run --release --package xtask --"
 
 [target.armv7-unknown-linux-gnueabi]
 linker = "arm-linux-gnueabi-gcc"