packaging: Add Nix files for creating development environment and the package

[imincik]

Add couple of Nix files which will allow following:

• Create full development environment using single command
• Run GRASS from any GitHub revision

Nix installation

• Install Nix
  (learn more about this installer)

curl --proto '=https' --tlsv1.2 -sSf \
    -L https://install.determinate.systems/nix \
    | sh -s -- install

GRASS development environment

• Run following command in source code root directory

nix develop

• Optionally, use direnv to activate the environment automatically when
  moving to the source code directory

echo "use flake" > .envrc
direnv allow

Running GRASS directly from the source code

• Run latest version from main branch

nix run github:OSGeo/grass#grass -- --text

• Run version from specific Git branch

nix run github:OSGeo/grass/<GIT-BRANCH>#grass -- --text

• Run version at specific Git revision

nix run github:OSGeo/grass/<GIT-REVISION>#grass -- --text

Usage documentation will be added to https://grasswiki.osgeo.org/wiki/Compile_and_Install

[imincik]

@neteler , I think, this PR is good to go now.

[echoix]

@wenzeslaus what PR prefix is appropriate? Should this one be added too? Nix? Build?

[imincik]

I changed commit prefix to packaging.

[echoix]

Perfect!

[imincik]

And here we go, nix run github:OSGeo/grass#grass (latest dev version) on my Ubuntu machine:

/cc @landam @wenzeslaus @neteler

[echoix]

Isn't the (exported) in the version expected to be filled in from git info since a couple of months ago? I think this is the fallback.

[imincik]

Isn't the (exported) in the version expected to be filled in from git info since a couple of months ago? I think this is the fallback.

I think this kind of version detection from git can't work for Nix package.

[echoix]

If git and the git repo is available at configure time, it should be detected. Otherwise there were some changes made this winter/early spring for helping with reproducible builds

[imincik]

If git and the git repo is available at configure time

That's the issue. Git is not available during the build.