ci: merge Go coverage reports before upload (#10666)

Attempting to fix the state of codecov action checks right now,
which are behaving very erratically.

Using the new functionality in Go 1.20 to merge multiple reports,
so now the unit & E2E coverage data reports are stored as artifacts
and then downloaded, merged, and finally uploaded to codecov as a
new job.

Additionally, add a `codecov.yml` config and try to turn down the
aggressiveness of it for CI checks.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

pkg/e2e/framework.go

@@ -20,6 +20,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"io/fs"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -134,7 +135,7 @@ func initializePlugins(t testing.TB, configDir string) {
 	require.NoError(t, os.MkdirAll(filepath.Join(configDir, "cli-plugins"), 0o755),
 		"Failed to create cli-plugins directory")
 	composePlugin, err := findExecutable(DockerComposeExecutableName)
-	if os.IsNotExist(err) {
+	if errors.Is(err, fs.ErrNotExist) {
 		t.Logf("WARNING: docker-compose cli-plugin not found")
 	}
 
@@ -161,20 +162,21 @@ func dirContents(dir string) []string {
 }
 
 func findExecutable(executableName string) (string, error) {
-	_, filename, _, _ := runtime.Caller(0)
-	root := filepath.Join(filepath.Dir(filename), "..", "..")
-	buildPath := filepath.Join(root, "bin", "build")
-
-	bin, err := filepath.Abs(filepath.Join(buildPath, executableName))
-	if err != nil {
-		return "", err
+	bin := os.Getenv("COMPOSE_E2E_BIN_PATH")
+	if bin == "" {
+		_, filename, _, _ := runtime.Caller(0)
+		buildPath := filepath.Join(filepath.Dir(filename), "..", "..", "bin", "build")
+		var err error
+		bin, err = filepath.Abs(filepath.Join(buildPath, executableName))
+		if err != nil {
+			return "", err
+		}
 	}
 
 	if _, err := os.Stat(bin); err == nil {
 		return bin, nil
 	}
-
-	return "", errors.Wrap(os.ErrNotExist, "executable not found")
+	return "", fmt.Errorf("looking for %q: %w", bin, fs.ErrNotExist)
 }
 
 func findPluginExecutable(pluginExecutableName string) (string, error) {