Replace !os.IsNotExist with == nil

!os.IsNotExist would also be true for other errors which don't mean the
file exists.

truststore_linux.go

@@ -25,12 +25,12 @@ var (
 
 func init() {
 	_, err := os.Stat("/etc/pki/ca-trust/source/anchors/")
-	if !os.IsNotExist(err) {
+	if err == nil {
 		SystemTrustFilename = "/etc/pki/ca-trust/source/anchors/mkcert-rootCA.pem"
 		SystemTrustCommand = []string{"update-ca-trust", "extract"}
 	} else {
 		_, err = os.Stat("/usr/local/share/ca-certificates/")
-		if !os.IsNotExist(err) {
+		if err == nil {
 			SystemTrustFilename = "/usr/local/share/ca-certificates/mkcert-rootCA.crt"
 			SystemTrustCommand = []string{"update-ca-certificates"}
 		}

truststore_nss.go

@@ -22,7 +22,7 @@ func init() {
 		"/Applications/Firefox Developer Edition.app",
 	} {
 		_, err := os.Stat(path)
-		hasNSS = hasNSS || !os.IsNotExist(err)
+		hasNSS = hasNSS || err == nil
 	}
 
 	switch runtime.GOOS {
@@ -34,7 +34,7 @@ func init() {
 		certutilPath = filepath.Join(strings.TrimSpace(string(out)), "bin", "certutil")
 
 		_, err = os.Stat(certutilPath)
-		hasCertutil = !os.IsNotExist(err)
+		hasCertutil = err == nil
 
 	case "linux":
 		var err error
@@ -90,7 +90,7 @@ func (m *mkcert) uninstallNSS() {
 
 func (m *mkcert) forEachNSSProfile(f func(profile string)) (found int) {
 	profiles, _ := filepath.Glob(FirefoxProfile)
-	if _, err := os.Stat(nssDB); !os.IsNotExist(err) {
+	if _, err := os.Stat(nssDB); err == nil {
 		profiles = append(profiles, nssDB)
 	}
 	if len(profiles) == 0 {
@@ -100,12 +100,12 @@ func (m *mkcert) forEachNSSProfile(f func(profile string)) (found int) {
 		if stat, err := os.Stat(profile); err != nil || !stat.IsDir() {
 			continue
 		}
-		if _, err := os.Stat(filepath.Join(profile, "cert9.db")); !os.IsNotExist(err) {
+		if _, err := os.Stat(filepath.Join(profile, "cert9.db")); err == nil {
 			f("sql:" + profile)
 			found++
 			continue
 		}
-		if _, err := os.Stat(filepath.Join(profile, "cert8.db")); !os.IsNotExist(err) {
+		if _, err := os.Stat(filepath.Join(profile, "cert8.db")); err == nil {
 			f("dbm:" + profile)
 			found++
 		}