*/*: change rust project's hardcoded target/release path [3/4]

See the previous 2nd commit to see more info

for the files changed in this commit, there wasn't any problem with how
the path was being handled per se

however, it was getting the path via

```
target/$(usex debug debug release)
```

but cargo_target_dir from cargo.eclass already exists. Hence, I changed
it to this so the ebuilds in the tree are more consistent

Signed-off-by: ingenarel (NeoJesus) <ingenarel_neojesus@disroot.org>
Part-of: https://codeberg.org/gentoo/guru/pulls/21
Signed-off-by: Michał Górny <mgorny@gentoo.org>

app-portage/gpkg/gpkg-1.4.0.ebuild

@@ -536,9 +536,7 @@ src_install() {
 	# would fail because gpkg-daemon doesn't define the GUI features).
 	# cargo.eclass honors USE=debug: target/debug/ when debug is set,
 	# target/release/ otherwise — see bug 975866.
-	local target_dir="target/$(usex debug debug release)"
-	dobin "${target_dir}/gpkg-daemon"
-	dobin "${target_dir}/gpkg"
+	dobin "$(cargo_target_dir)/"gpkg{,-daemon}
 
 	# D-Bus system bus configuration
 	insinto /etc/dbus-1/system.d

media-sound/spotify-adblock/spotify-adblock-1.0.1.ebuild

@@ -59,7 +59,7 @@ src_install(){
 	dobin "${T}/spotify-adblock"
 
 	# library
-	newlib.so target/$(usex debug debug release)/libspotifyadblock.so ${PN}.so
+	newlib.so "$(cargo_target_dir)/libspotifyadblock.so" "${PN}.so"
 
 	# config
 	insinto "/etc/${PN}"

media-sound/spotify-adblock/spotify-adblock-1.0.2.ebuild

@@ -59,7 +59,7 @@ src_install(){
 	dobin "${T}/spotify-adblock"
 
 	# library
-	newlib.so target/$(usex debug debug release)/libspotifyadblock.so ${PN}.so
+	newlib.so "$(cargo_target_dir)/libspotifyadblock.so" "${PN}.so"
 
 	# config
 	insinto "/etc/${PN}"

media-sound/spotify-adblock/spotify-adblock-1.0.3.ebuild

@@ -63,7 +63,7 @@ src_install(){
 	dobin "${T}/spotify-adblock"
 
 	# library
-	newlib.so target/$(usex debug debug release)/libspotifyadblock.so ${PN}.so
+	newlib.so "$(cargo_target_dir)/libspotifyadblock.so" "${PN}.so"
 
 	# config
 	insinto "/etc/${PN}"