Unexpected result using sys_get_shell in argument of hypersetup

I would like to include the content of an environment variable inside a PDFs metadata. (More specifically the commit inside of a CI pipeline). To do this I used an old(ish) macro of mine to get the value of environment variables using sys_get_shell:nnN + kpsewhich --var-value.

Surprisingly this works when storing the result inside an intermediate variable but not when using the output directly. The screenshot shows the metadata information as reported by TeXstudio.

MWE:

documentclass{article}

usepackage{hyperref}
usepackage{xparse}

ExplSyntaxOn

NewDocumentCommand getenv { o m }
{
    sys_get_shell:nnN { kpsewhich ~ --var-value ~ #2 } { } l_tmpa_tl
    tl_trim_spaces:N l_tmpa_tl
    IfNoValueTF { #1 }{
        tl_to_str:N l_tmpa_tl
    }{
        tl_set_eq:NN #1 l_tmpa_tl
    }
}

ExplSyntaxOff

getenv[LANG]{LANG}
hypersetup{
    pdfinfo={
        LangIndirect=LANG,
        LangDirect=getenv{LANG},
    },
}

begin{document}

ExplSyntaxOn
tl_to_str:N l_tmpa_tl
ExplSyntaxOff

end{document}