bug#41244: xdg-open: mailto doesnt work

[Jonathan Brielmaier]

Trying `xdg-open mailto:address@hidden` in XFCE or MATE fails. The
same happens when you click on a mailto link in Icecat. I have
claws-mail and icedove installed. They both have
`MimeType=x-scheme-handler/mailto;` in their desktop files.

```
jonathan@x250 ~$ Can't locate URI/Escape.pm in @INC (you may need to
install the URI::Escape module) (@INC contains:
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/site_perl/5.30.2/x86_64-linux-thread-multi
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/site_perl/5.30.2
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/5.30.2/x86_64-linux-thread-multi
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/5.30.2)
at
/gnu/store/rxjzd5v68ia80ksxhx860x9nli2hg57p-exo-0.12.11/lib/xfce4/exo/exo-compose-mail
line 24.
BEGIN failed--compilation aborted at
/gnu/store/rxjzd5v68ia80ksxhx860x9nli2hg57p-exo-0.12.11/lib/xfce4/exo/exo-compose-mail
line 24.
Can't locate URI/Escape.pm in @INC (you may need to install the
URI::Escape module) (@INC contains:
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/site_perl/5.30.2/x86_64-linux-thread-multi
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/site_perl/5.30.2
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/5.30.2/x86_64-linux-thread-multi
/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib/perl5/5.30.2)
at
/gnu/store/rxjzd5v68ia80ksxhx860x9nli2hg57p-exo-0.12.11/lib/xfce4/exo/exo-compose-mail
line 24.
BEGIN failed--compilation aborted at
/gnu/store/rxjzd5v68ia80ksxhx860x9nli2hg57p-exo-0.12.11/lib/xfce4/exo/exo-compose-mail
line 24.
```

Installing perl-uri-escape does not resolve the issue. Neither adding it
as input or propagated-input to exo.