lightningd: fix another trivial memleak when running installed version.

[rustyrussell]

**BROKEN** lightningd: MEMLEAK: 0x55ff8e8ec268
**BROKEN** lightningd:   label=char[]
**BROKEN** lightningd:   alloc:
**BROKEN** lightningd:     ccan/ccan/tal/tal.c:488 (tal_alloc_)
**BROKEN** lightningd:     ccan/ccan/tal/tal.c:517 (tal_alloc_arr_)
**BROKEN** lightningd:     ccan/ccan/tal/str/str.c:81 (tal_vfmt_)
**BROKEN** lightningd:     ccan/ccan/tal/str/str.c:37 (tal_fmt_)
**BROKEN** lightningd:     lightningd/lightningd.c:557 (find_subdaemons_and_plugins)
**BROKEN** lightningd:     lightningd/lightningd.c:1196 (main)
**BROKEN** lightningd:   parents:
**BROKEN** lightningd:     struct plugins

Changelog-None: Introduced this release. Fixes: https://github.com/ElementsProject/lightning/issues/7590

[ShahanaFarooqui]

ACK dd2554fcdc9f6e33de0f74f4e659b1ef499cf2ec