gnu: et: Don't use unstable tarball.

This follows up commit ae113b976b (what
looked like a build error was transient file system unrelatedness.)

* gnu/packages/ssh.scm (et)[source]: Use GIT-FETCH and
GIT-FILE-NAME.

gnu/packages/ssh.scm

@@ -442,12 +442,12 @@ especially over Wi-Fi, cellular, and long-distance links.")
     (version "3.1.0")
     (source
      (origin
-       (method url-fetch)
+       (method git-fetch)
-       (uri (string-append
+       (uri (git-reference
-             "https://github.com/MisterTea/EternalTCP/archive/et-v"
+             (url "https://github.com/MisterTea/EternalTCP.git")
-             version ".tar.gz"))
+             (commit (string-append "et-v" version))))
        (sha256
-        (base32 "0k34l33ns2i2fijvw6ypxjmjcih11ydi9ha84jr8vdx8ii7p8p11"))))
+        (base32 "1m5caxckn2ihwp9s2pbyh5amxlpwr7yc54q8s0kb10fr52w2vfnm"))))
     (build-system cmake-build-system)
     (arguments `(#:tests? #f))
     (native-inputs