add flake support

[maxhbr]

This is currently failing to build the license-expression dependency:

error: builder for '/nix/store/hadzd22512dpsbq4l56rc32r0y2lsidr-python3.10-license-expression-30.1.0.drv' failed with exit code 1;
       last 10 log lines:
       > source root is license-expression-30.1.0
       > setting SOURCE_DATE_EPOCH to timestamp 1673980441 of file license-expression-30.1.0/setup.cfg
       > patching sources
       > Removing path dependencies
       > Finished removing path dependencies
       > Removing git dependencies
       > Finished removing git dependencies
       > configuring
       > configure flags: --prefix=/nix/store/62d35g1kn8fms5w3gm4qsf57lrffnx5v-python3.10-license-expression-30.1.0
       > /nix/store/kk3qbkyrski18j8fwrpd5dgvplbdz7b2-stdenv-linux/setup: line 1289: ./configure: cannot execute: required file not found
       For full logs, run 'nix log /nix/store/hadzd22512dpsbq4l56rc32r0y2lsidr-python3.10-license-expression-30.1.0.drv'.
error: 1 dependencies of derivation '/nix/store/5dmv6fwxam2rzg6cy2k1ffl8r7hr9n82-python3.10-spdx-tools-0.8.0a1.drv' failed to build
error: 1 dependencies of derivation '/nix/store/z4jw2ckrkfs66l3ch4xhr7n09qjg9kv9-python3.10-opossum-lib-py-0.1.drv' failed to build