Avoid allocations with (*regexp.Regexp).MatchString

We should use `(*regexp.Regexp).MatchString` instead of
`(*regexp.Regexp).Match([]byte(...))` when matching string to avoid
unnecessary `[]byte` conversions and reduce allocations.

Example benchmark:

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

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

goos: linux
goarch: amd64
pkg: go.ddosify.com/ddosify/core/types
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 4542174	       277.2 ns/op	       8 B/op	       1 allocs/op
BenchmarkMatchString-16    	 7210927	       172.1 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	go.ddosify.com/ddosify/core/types	3.908s

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

core/types/scenario.go

@@ -86,7 +86,7 @@ func (s *Scenario) validate() error {
 
 	// add global envs
 	for key := range s.Envs {
-		if !envVarNameRegexp.Match([]byte(key)) { // not a valid env definition
+		if !envVarNameRegexp.MatchString(key) { // not a valid env definition
 			return fmt.Errorf("env key is not valid: %s", key)
 		}
 		definedEnvs[key] = struct{}{} // exist
@@ -98,7 +98,7 @@ func (s *Scenario) validate() error {
 			return fmt.Errorf("csv key can not have dot in it: %s", key)
 		}
 		for _, s := range splitted {
-			if !envVarNameRegexp.Match([]byte(s)) { // not a valid env definition
+			if !envVarNameRegexp.MatchString(s) { // not a valid env definition
 				return fmt.Errorf("csv key is not valid: %s", key)
 			}
 		}
@@ -112,7 +112,7 @@ func (s *Scenario) validate() error {
 
 		// enrich Envs map with captured envs from each step
 		for _, ce := range st.EnvsToCapture {
-			if !envVarNameRegexp.Match([]byte(ce.Name)) { // not a valid env definition
+			if !envVarNameRegexp.MatchString(ce.Name) { // not a valid env definition
 				return fmt.Errorf("captured env key is not valid: %s", ce.Name)
 			}
 			definedEnvs[ce.Name] = struct{}{}