bug#50901: perl6-tap-harness: perl6 path issue

[Christopher Howard]

Hello. I'm not a Perl expert, but I was needing to use the prove6
command from perl6-tap-harness. When I set up "guix environment --ad-
hoc perl6-tap-harness rakudo" and run the "prove6" command, I get this
error:

```
/gnu/store/qm5p9yqaal1lppcag8ag69wwhj2q0rjg-profile/bin/prove6:
/gnu/store/7lacqfg5rqnjdnwhva0n4hiw16gwfa2h-perl6-tap-harness-
0.0.7/bin/.prove6-real: perl6: bad interpreter: No such file or
directory
````

But I don't have trouble running perl6 command by itself.

To save some trouble, here is the prove6 script and the head of
.prove6-real:

```
$ head /gnu/store/7lacqfg5rqnjdnwhva0n4hiw16gwfa2h-perl6-tap-harness-
0.0.7/bin/prove6 
#!/gnu/store/pwcp239kjf7lnj5i4lkdzcfcxwcfyk72-bash-minimal-
5.0.16/bin/bash
export PERL6LIB="/gnu/store/7lacqfg5rqnjdnwhva0n4hiw16gwfa2h-perl6-tap-
harness-
0.0.7/share/perl6/lib,/gnu/store/7lacqfg5rqnjdnwhva0n4hiw16gwfa2h-
perl6-tap-harness-
0.0.7/share/perl6/site/lib,/gnu/store/7lacqfg5rqnjdnwhva0n4hiw16gwfa2h-
perl6-tap-harness-
0.0.7/share/perl6/vendor/lib,/gnu/store/db59gi37wfp8rg4h150qhlh65lyb300
a-rakudo-2019.03.1/share/perl6/lib${PERL6LIB:+,}$PERL6LIB"
exec -a "$0" "/gnu/store/7lacqfg5rqnjdnwhva0n4hiw16gwfa2h-perl6-tap-
harness-0.0.7/bin/.prove6-real" "$@"

$ head /gnu/store/7lacqfg5rqnjdnwhva0n4hiw16gwfa2h-perl6-tap-harness-
0.0.7/bin/.prove6-real
#! perl6

use TAP;

multi sub listall(IO::Path $path where .d) {
    for $path.dir(:test{ !$^file.starts-with('.') }) -> $entry {
        listall($entry);
    }
}
multi listall(IO::Path $path) {
```

My system information:

```
christopher@theoden ~$ neofetch --stdout
christopher@theoden 
------------------- 
OS: Guix System 8313587f612bdbfe0116288afaa534b181ac7218 x86_64 
Host: OptiPlex 9020 00 
Kernel: 5.13.16-gnu1 
Uptime: 7 days, 1 hour, 44 mins 
Packages: 95 (guix-system), 120 (guix-user) 
Shell: bash 5.0.16 
Resolution: 1920x1080 
DE: GNOME 
Theme: Adwaita [GTK2/3] 
Icons: Adwaita [GTK2/3] 
Terminal: .gnome-terminal 
CPU: Intel i5-4570 (4) @ 3.600GHz 
GPU: Intel HD Graphics 
GPU: AMD ATI Radeon HD 8490 / R5 235X OEM 
Memory: 4622MiB / 7869MiB 
```

-- 
Christopher Howard
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