Fix pidgin-with-plugins derivation

```sh
$ pidgin --version
Pidgin 2.10.10 (libpurple 2.10.10)
```

Actual content of pidgin-with-plugins installed:
``` sh
export PURPLE_PLUGIN_PATH=$PURPLE_PLUGIN_PATH${PURPLE_PLUGIN_PATH:+:}/nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/purple-2.10.10
export PURPLE_PLUGIN_PATH=$PURPLE_PLUGIN_PATH${PURPLE_PLUGIN_PATH:+:}/nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/pidgin
exec
/nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/bin/.pidgin-wrapped
"${extraFlagsArray[@]}" "$@"
```
So the PURPLE_PLUGIN_PATH looks into something that does not exist...

```sh
$ file /nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/purple-2.10.10
/nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/purple-2.10.10: cannot open `/nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/purple-2.10.10' (No such file or directory)

$ file /nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/purple-2
/nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/purple-2: directory
```

Because the folder created is named pidgin-2 (what I called
majorVersion) and not pidgin-2.10.10 which is the default version.
(thus resulting of no plugins loaded after installation, pidgin-sipe for 1):

``` sh
$ ll /nix/store/ifbf01z4w6pji601jxcsrd034lh60r0g-pidgin-with-plugins-2.10.10/lib/
total 76K
lrwxrwxrwx 1 root nixbld   68 Jan  1  1970 finch -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/finch
dr-xr-xr-x 2 root nixbld 4.0K Jan  1  1970 gdk-pixbuf-loaders-2.0
lrwxrwxrwx 1 root nixbld   66 Jan  1  1970 gnt -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/gnt
lrwxrwxrwx 1 root nixbld   72 Jan  1  1970 libgnt.la -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libgnt.la
lrwxrwxrwx 1 root nixbld   72 Jan  1  1970 libgnt.so -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libgnt.so
lrwxrwxrwx 1 root nixbld   74 Jan  1  1970 libgnt.so.0 -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libgnt.so.0
lrwxrwxrwx 1 root nixbld   79 Jan  1  1970 libgnt.so.0.8.10 -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libgnt.so.0.8.10
lrwxrwxrwx 1 root nixbld   82 Jan  1  1970 libpurple-client.la -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple-client.la
lrwxrwxrwx 1 root nixbld   82 Jan  1  1970 libpurple-client.so -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple-client.so
lrwxrwxrwx 1 root nixbld   84 Jan  1  1970 libpurple-client.so.0 -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple-client.so.0
lrwxrwxrwx 1 root nixbld   90 Jan  1  1970 libpurple-client.so.0.10.10 -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple-client.so.0.10.10
lrwxrwxrwx 1 root nixbld   75 Jan  1  1970 libpurple.la -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple.la
lrwxrwxrwx 1 root nixbld   75 Jan  1  1970 libpurple.so -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple.so
lrwxrwxrwx 1 root nixbld   77 Jan  1  1970 libpurple.so.0 -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple.so.0
lrwxrwxrwx 1 root nixbld   83 Jan  1  1970 libpurple.so.0.10.10 -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/libpurple.so.0.10.10
lrwxrwxrwx 1 root nixbld   68 Jan  1  1970 perl5 -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/perl5
dr-xr-xr-x 2 root nixbld 4.0K Jan  1  1970 pidgin
lrwxrwxrwx 1 root nixbld   72 Jan  1  1970 pkgconfig -> /nix/store/kd55fbz9jvh5vnjbhms7r8460cp4anhg-pidgin-2.10.10/lib/pkgconfig
dr-xr-xr-x 2 root nixbld 4.0K Jan  1  1970 purple-2
```