Fixes to build and install opam master in Windows

CHANGES.md

@@ -1,7 +1,8 @@
 unreleased
 ----------
 
-- Build and install opam master from source in Windows images. (@MisterDA #140, #142)
+- Build and install opam master from source in Windows images.
+  (@MisterDA #140, #142, #143)
 - Include the ocaml-beta-repository in the images. (@kit-ty-kate #132, review by @MisterDA)
 - Add OpenSUSE 15.4, deprecate OpenSUSE 15.3. (@MisterDA #138)
 - Update to bubblewrap 0.7.0. (@MisterDA #131)

src-opam/opam.ml

@@ -67,7 +67,7 @@ let install_opam_from_source_windows ?cyg ?prefix
   (* Docker doesn't allow copying executables from /usr/local/bin
      (why??), so tar the opam installation. *)
   @@ Windows.Cygwin.run_sh ?cyg
-       {|cd /usr/local/bin && tar -cf /cygdrive/c/TEMP/opam.tar .|}
+       {|cd /usr/local/bin && tar -cf /cygdrive/c/opam.tar .|}
 
 let bubblewrap_minimum = (0, 4, 1)
 let bubblewrap_latest = (0, 7, 0)
@@ -276,8 +276,7 @@ let copy_opams ~src ~dst opam_branches =
 (* FIXME: only support building opam master for now *)
 let copy_opams_windows = function
   | [ { public_name; aliases; _ } ] ->
-      copy ~from:"opam-builder" ~src:[ {|C:\TEMP\opam.tar|} ] ~dst:{|C:\TEMP\|}
-        ()
+      copy ~from:"opam-builder" ~src:[ {|C:\opam.tar|} ] ~dst:{|C:\TEMP\|} ()
       @@ run
            {|C:\cygwin64\bin\tar.exe -xf /cygdrive/c/TEMP/opam.tar -C /usr/local/bin && del C:\TEMP\opam.tar|}
       @@ run