backblaze-b2: Fix autocompletion support

backblaze v1.3.6 incorporates[1] a fix[2] for autocompletion that replaced
`have` function with `_have`. The postInstall hook therefore needs
to be updated to replace `_have`.

[1] https://github.com/Backblaze/B2_Command_Line_Tool#136-august-21-2018
[2] Backblaze/B2_Command_Line_Tool#497

pkgs/development/tools/backblaze-b2/default.nix

@@ -33,7 +33,7 @@ buildPythonApplication rec {
   postInstall = ''
     mv "$out/bin/b2" "$out/bin/backblaze-b2"
 
-    sed 's/^have b2 \&\&$/_have backblaze-b2 \&\&/'  -i contrib/bash_completion/b2
+    sed 's/^_have b2 \&\&$/_have backblaze-b2 \&\&/'  -i contrib/bash_completion/b2
     sed 's/^\(complete -F _b2\) b2/\1 backblaze-b2/' -i contrib/bash_completion/b2
 
     mkdir -p "$out/etc/bash_completion.d"