COIN: Enable setting Debian target distro with env variable

Change-Id: I8cc0e9127ee7dd1e810812688ad32e759510cbe9
Reviewed-by: Toni Saario <toni.saario@qt.io>
(cherry picked from commit d4568bc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

coin/instructions/debian/debian_build_module.yaml

@@ -104,7 +104,7 @@ instructions:
   - type: ExecuteCommand
     command: ["sbuild",
       "--build-dep-resolver=apt",
-      "-sAd", "tqtc-focal",
+      "-sAd", "{{.Env.COIN_TARGET_DIST}}",
       "-c", "{{.Env.COIN_SBUILD_CHROOT}}",
       "--build-dir", "output/debian_packages",
       "--extra-repository={{.Env.COIN_EXTRA_DEBIAN_REPO}}",

coin/instructions/debian/prepare_debian_env.yaml

@@ -4,6 +4,13 @@ enable_if:
   property: features
   contains_value: DebianPackaging
 instructions:
+  - type: EnvironmentVariable
+    variableName: COIN_TARGET_DIST
+    variableValue: "tqtc-focal"
+    enable_if:
+      condition: runtime
+      env_var:  COIN_TARGET_DIST
+      equals_value: null
   - type: EnvironmentVariable
     variableName: COIN_SBUILD_CHROOT
     variableValue: "stable-arm64-sbuild"