golangci: disable gosec linter in martian dir

Some of the issues need some discussion before the fix:

internal/martian/proxy.go: G402: TLS MinVersion too low. (gosec)
       return &tls.Config{}

internal/martian/mitm/mitm.go: G505: Blocklisted import crypto/sha1: weak cryptographic primitive (gosec)
        "crypto/sha1"

internal/martian/mitm/mitm.go: G401: Use of weak cryptographic primitive (gosec)
        h := sha1.New()

.golangci.yml

@@ -122,6 +122,7 @@ issues:
         - goheader
         - gochecknoglobals
         - errcheck
+        - gosec
 
     - linters:
         - thelper