Allow version-type: strict + otp-version: master for Elixir -based builds (#144)

* Check tests failing for something that should pass

* Test with version-type: strict

* Add more tests

GitHub actions is refusing to run a given workflow :shrug

* Try to fix new test that shouldn't be failing

Which is exactly what we intended to originate by starting a new
pull request.

* Fix tests as per CI

* Fix tests as per CI

* Adapt to "latest"

* Fix as per CI results

Actions' window shows everything as Ok but then internals not

Error when evaluating 'runs-on' for job 'integration_test'. .github/workflows/ubuntu.yml (Line: 19, Col: 14): Unexpected type of value '', expected type: OneOf.

Strange one, this one!

* Try to "help" for non-OTP declared input

We do this for OTP-, but not for maint-
as the former is hopefully more common
than the latter

* Introduce `latest` to signify `master`

Semantically they're similar, but with `latest` we know:
1. to fetch Elixir's no-otp-... version
2. to fetch Erlang's master

whereas with 25 (latest at this moment) we'd
try to fetch elixirvsn-otp-25 which could fail with
Elixir master and version-type strict

* Fix as per CI

Still not 100% convinced that `latest` should be separate from `master`
but I want to know how tests run, in this case in particular

* Fix as per CI

* Fix as per CI

* Revert potential wrong decision

* Fix as per understand recent Elixir changes (namely master v. main)

* Try to simplify it

We introduce the concept of version v. branch (as per @ericmj)
and we use that to find the most appropriate Erlang/Elixir combo.

.github/workflows/ubuntu.yml

@@ -21,6 +21,13 @@ jobs:
       fail-fast: false
       matrix:
         combo:
+          - otp-version: 'master'
+            elixir-version: '1.14.0'
+            os: 'ubuntu-latest'
+          - otp-version: 'master'
+            elixir-version: '1.14.0'
+            os: 'ubuntu-latest'
+            version-type: 'strict'
           - otp-version: '25'
             elixir-version: '1'
             rebar3-version: '3'
@@ -113,12 +120,13 @@ jobs:
           - elixir-version: 'master'
             otp-version: '23.1'
             os:  'ubuntu-20.04'
-          - elixir-version: 'master'
+          - elixir-version: 'main'
             otp-version: '25'
             os:  'ubuntu-20.04'
             version-type: 'strict'
           - gleam-version: 'nightly'
             otp-version: '24'
+            os: 'ubuntu-latest'
           - gleam-version: '0.23'
             otp-version: '24'
             os: 'ubuntu-latest'