[CI] Add back matrix CI script (#2916)

Adds back the matrix CI script which I accidentally deleted in a
previous PR.

scripts/check-matrix-job.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the SwiftNIO open source project
+##
+## Copyright (c) 2024 Apple Inc. and the SwiftNIO project authors
+## Licensed under Apache License v2.0
+##
+## See LICENSE.txt for license information
+## See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+##
+## SPDX-License-Identifier: Apache-2.0
+##
+##===----------------------------------------------------------------------===##
+set -euo pipefail
+
+log() { printf -- "** %s\n" "$*" >&2; }
+error() { printf -- "** ERROR: %s\n" "$*" >&2; }
+fatal() { error "$@"; exit 1; }
+
+test -n "${SWIFT_VERSION:-}" || fatal "SWIFT_VERSION unset"
+test -n "${COMMAND:-}" || fatal "COMMAND unset"
+swift_version="$SWIFT_VERSION"
+command="$COMMAND"
+command_5_8="$COMMAND_OVERRIDE_5_8"
+command_5_9="$COMMAND_OVERRIDE_5_9"
+command_5_10="$COMMAND_OVERRIDE_5_10"
+command_nightly_6_0="$COMMAND_OVERRIDE_NIGHTLY_6_0"
+command_nightly_main="$COMMAND_OVERRIDE_NIGHTLY_MAIN"
+
+if [[ "$swift_version" == "5.8" ]] && [[ -n "$command_5_8" ]]; then
+  log "Running 5.8 command override"
+  eval "$command_5_8"
+elif [[ "$swift_version" == "5.9" ]] && [[ -n "$command_5_9" ]]; then
+  log "Running 5.9 command override"
+  eval "$command_5_9"
+elif [[ "$swift_version" == "5.10" ]] && [[ -n "$command_5_10" ]]; then
+  log "Running 5.10 command override"
+  eval "$command_5_10"
+elif [[ "$swift_version" == "nightly-6.0" ]] && [[ -n "$command_nightly_6_0" ]]; then
+  log "Running nightly 6.0 command override"
+  eval "$command_nightly_6_0"
+elif [[ "$swift_version" == "nightly-main" ]] && [[ -n "$command_nightly_main" ]]; then
+  log "Running nightly main command override"
+  eval "$command_nightly_main"
+else
+  log "Running default command"
+  eval "$command"
+fi