Add rudimentary grep for runtime warnings in CI #625

Job	Run time
[23.3/1.14.5] new Phoenix app analysed by Credo [OTP/Elixir]	47s
[24.3/1.14.5] new Phoenix app analysed by Credo [OTP/Elixir]	37s
[25.3/1.15.7] new Phoenix app analysed by Credo [OTP/Elixir]	35s
[25.3/1.16.2] new Phoenix app analysed by Credo [OTP/Elixir]	33s
[26.1/1.15.7] new Phoenix app analysed by Credo [OTP/Elixir]	37s
[26.1/1.16.2] new Phoenix app analysed by Credo [OTP/Elixir]	37s
Elixir 1.13.4 OTP 24.3 - v1.6	39s
Elixir 1.14.5 OTP 24.3 - v1.6	37s
Elixir 1.13.4 OTP 25.3 - v1.6	39s
Elixir 1.15.7 OTP 25.3 - v1.6	36s
Elixir 1.16.2 OTP 25.3 - v1.6	35s
Elixir 1.17.3 OTP 25.3 - v1.6	36s
Elixir 1.15.7 OTP 26.2 - v1.6	35s
Elixir 1.16.2 OTP 26.2 - v1.6	40s
Elixir 1.17.3 OTP 26.2 - v1.6	38s
Elixir 1.13.4 OTP 24.3 - main	37s
Elixir 1.14.5 OTP 24.3 - main	36s
Elixir 1.13.4 OTP 25.3 - main	40s
Elixir 1.15.7 OTP 25.3 - main	35s
Elixir 1.16.2 OTP 25.3 - main	37s
Elixir 1.17.3 OTP 25.3 - main	33s
Elixir 1.15.7 OTP 26.2 - main	39s
Elixir 1.16.2 OTP 26.2 - main	38s
Elixir 1.17.3 OTP 26.2 - main	36s
	14m 52s

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