feat: bounded concurrency for binary inspection

Uses a semaphore to bound concurrency to GOMAXPROCS value. Fixes #14.

On my system, a cached --dry-run goes from ~820ms to ~130ms, and an
uncached one that needs to hit the network goes from ~2s to ~300ms.

Benchmark results from my laptop:
```
$ hyperfine --warmup 1 'go-global-update --dry-run' './go-global-update-fork --dry-run'
Benchmark 1: go-global-update --dry-run
  Time (mean ± σ):     821.7 ms ±   7.7 ms    [User: 171.8 ms, System: 139.0 ms]
  Range (min … max):   806.2 ms … 832.8 ms    10 runs

Benchmark 2: ./go-global-update-fork --dry-run
  Time (mean ± σ):     129.5 ms ±   4.2 ms    [User: 152.8 ms, System: 187.8 ms]
  Range (min … max):   120.3 ms … 137.3 ms    22 runs
```

Results with no modcache (cleared via `go clean -modcache`):
```
hyperfine --prepare 'go clean -modcache' 'go-global-update --dry-run' './go-global-update-fork --dry-run'
Benchmark 1: go-global-update --dry-run
  Time (mean ± σ):      1.936 s ±  0.072 s    [User: 0.236 s, System: 0.243 s]
  Range (min … max):    1.875 s …  2.120 s    10 runs

Benchmark 2: ./go-global-update-fork --dry-run
  Time (mean ± σ):     303.4 ms ±  22.4 ms    [User: 193.1 ms, System: 261.4 ms]
  Range (min … max):   284.7 ms … 361.6 ms    10 runs
```

internal/gobinaries/introspect_binaries.go

@@ -1,26 +1,42 @@
 package gobinaries
 
-import "fmt"
+import (
+	"fmt"
+	"runtime"
+	"sync"
+)
 
 type IntrospectionResult struct {
 	Binary GoBinary
 	Error  error
 }
 
 func IntrospectBinaries(introspecter *Introspecter, binaryNames []string) []IntrospectionResult {
-	var results []IntrospectionResult
-
-	for _, binaryName := range binaryNames {
-		binary, err := introspecter.Introspect(binaryName)
-		if err != nil {
-			err = fmt.Errorf("could not introspect binary %s: %w", binaryName, err)
-		}
-
-		results = append(results, IntrospectionResult{
-			Binary: binary,
-			Error:  err,
-		})
+	results := make([]IntrospectionResult, len(binaryNames))
+
+	var wg sync.WaitGroup
+	semaphore := make(chan struct{}, runtime.GOMAXPROCS(0))
+	for i, binaryName := range binaryNames {
+		i, binaryName := i, binaryName
+
+		wg.Add(1)
+		semaphore <- struct{}{}
+		go func() {
+			defer wg.Done()
+			defer func() { <-semaphore }()
+
+			binary, err := introspecter.Introspect(binaryName)
+			if err != nil {
+				err = fmt.Errorf("could not introspect binary %s: %w", binaryName, err)
+			}
+
+			results[i] = IntrospectionResult{
+				Binary: binary,
+				Error:  err,
+			}
+		}()
 	}
+	wg.Wait()
 
 	return results
 }