Add builds for sr.ht and multi platform tests

.builds/alpine.yaml

+image: alpine/latest
+packages:
+  - elixir
+  - file-dev
+sources:
+  - https://git.sr.ht/~href/gen_magic
+tasks:
+  - setup: |
+      mix local.hex --force
+  - build: |
+      cd gen_magic
+      mix deps.get
+      MIX_ENV=test mix compile
+  - test: |
+      cd gen_magic
+      mix test
+

.builds/archlinux.yaml

+image: archlinux
+packages:
+  - elixir
+  - file
+sources:
+  - https://git.sr.ht/~href/gen_magic
+tasks:
+  - setup: |
+      mix local.hex --force
+  - build: |
+      cd gen_magic
+      mix deps.get
+      MIX_ENV=test mix compile
+  - test: |
+      cd gen_magic
+      mix test
+

.builds/debian-oldstable.yaml

+image: debian/oldstable
+packages:
+  - build-essential
+  - erlang
+  - erlang-dev
+  - elixir
+  - libmagic-dev
+sources:
+  - https://git.sr.ht/~href/gen_magic
+tasks:
+  - setup: |
+      mix local.hex --force
+  - build: |
+      cd gen_magic
+      mix deps.get
+      MIX_ENV=test mix compile
+  - test: |
+      cd gen_magic
+      mix test
+

.builds/debian-stable.yaml

+image: debian/stable
+packages:
+  - build-essential
+  - erlang
+  - erlang-dev
+  - elixir
+  - libmagic-dev
+sources:
+  - https://git.sr.ht/~href/gen_magic
+tasks:
+  - setup: |
+      mix local.hex --force
+  - build: |
+      cd gen_magic
+      mix deps.get
+      MIX_ENV=test mix compile
+  - test: |
+      cd gen_magic
+      mix test
+

.builds/debian-testing.yaml

+image: debian/testing
+packages:
+  - build-essential
+  - erlang
+  - erlang-dev
+  - elixir
+  - libmagic-dev
+sources:
+  - https://git.sr.ht/~hrefhref/gen_magic
+tasks:
+  - setup: |
+      mix local.hex --force
+  - build: |
+      cd gen_magic
+      mix deps.get
+      MIX_ENV=test mix compile
+  - test: |
+      cd gen_magic
+      mix test
+

.builds/fedora-latest.yaml

+image: fedora/latest
+packages:
+  - make
+  - gcc
+  - kernel-devel
+  - erlang
+  - elixir
+  - file-devel
+sources:
+  - https://git.sr.ht/~href/gen_magic
+tasks:
+  - setup: |
+      sudo dnf -y group install 'Development Tools'
+      mix local.hex --force
+  - build: |
+      cd gen_magic
+      mix deps.get
+      MIX_ENV=test mix compile
+  - test: |
+      cd gen_magic
+      mix test
+

.builds/freebsd.yaml

+image: freebsd/latest
+packages:
+  - elixir
+  - gmake
+sources:
+  - https://git.sr.ht/~href/gen_magic
+tasks:
+  - setup: |
+      mix local.hex --force
+  - build: |
+      cd gen_magic
+      mix deps.get
+      MIX_ENV=test mix compile
+  - test: |
+      cd gen_magic
+      mix test
+