main/postmarketos-installkernel: install to /sbin (MR 5728)

This reverts 62214096, which basically broke installkenrel because
the kernel build system is hardcoded to look for installkernel in /
sbin/installkernel.

We'll have to move then once usr merge is complete and the symlink is
created for /sbin
[ci:skip-build]: already built successfully in CI

main/postmarketos-installkernel/APKBUILD

@@ -4,7 +4,7 @@
 # https://gitlab.alpinelinux.org/alpine/aports/-/tree/master/main/installkernel
 pkgname=postmarketos-installkernel
 pkgver=0.1
-pkgrel=2
+pkgrel=3
 pkgdesc="postmarketOS script for installing mainline kernels"
 url="https://gitlab.postmarketos.org/postmarketOS/pmaports/tree/main/postmarketos-installkernel"
 arch="noarch"
@@ -20,7 +20,7 @@ build() {
 }
 
 package() {
-	install -Dm755 "$srcdir"/installkernel-pmos "$pkgdir"/usr/bin/installkernel
+	install -Dm755 "$srcdir"/installkernel-pmos "$pkgdir"/sbin/installkernel
 }
 
 sha512sums="