refactor(tmpl): avoid unnecessary byte/string conversion (#4356)

We can use `(*regexp.Regexp).MatchString` instead of
`(*regexp.Regexp).Match([]byte(...))` to avoid unnecessary `[]byte`
conversions and reduce allocations. A one-line change for free
performance gain.

Benchmark:

```go
func BenchmarkMatch(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := envOnlyRe.Match([]byte("{{ .Env.FOO }}")); !match {
			b.Fail()
		}
	}
}

func BenchmarkMatchString(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := envOnlyRe.MatchString("{{ .Env.FOO }}"); !match {
			b.Fail()
		}
	}
}
```

Result:

```
goos: linux
goarch: amd64
pkg: github.com/goreleaser/goreleaser/internal/tmpl cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 4320873	       381.2 ns/op	      16 B/op	       1 allocs/op
BenchmarkMatchString-16    	 5973543	       203.9 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/goreleaser/goreleaser/internal/tmpl	3.366s
```

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

internal/tmpl/tmpl.go

@@ -260,6 +260,8 @@ func (e ExpectedSingleEnvErr) Error() string {
 	return "expected {{ .Env.VAR_NAME }} only (no plain-text or other interpolation)"
 }
 
+var envOnlyRe = regexp.MustCompile(`^{{\s*\.Env\.[^.\s}]+\s*}}$`)
+
 // ApplySingleEnvOnly enforces template to only contain a single environment variable
 // and nothing else.
 func (t *Template) ApplySingleEnvOnly(s string) (string, error) {
@@ -272,8 +274,7 @@ func (t *Template) ApplySingleEnvOnly(s string) (string, error) {
 	// but regexp reduces the complexity and should be sufficient,
 	// given the context is mostly discouraging users from bad practice
 	// of hard-coded credentials, rather than catch all possible cases
-	envOnlyRe := regexp.MustCompile(`^{{\s*\.Env\.[^.\s}]+\s*}}$`)
-	if !envOnlyRe.Match([]byte(s)) {
+	if !envOnlyRe.MatchString(s) {
 		return "", ExpectedSingleEnvErr{}
 	}