Remove temp dir created when downloading FTL

[yubiuser]

What does this PR aim to accomplish?:

When installing FTL, the binary is downloaded in a temporary directory. This can cause space issues on devices with small /tmp systems when a FTL is frequently installed (e.g. by checking out different FTL branches).

nanopi@nanopi:~$ df -h
Dateisystem    Größe Benutzt Verf. Verw% Eingehängt auf
udev            422M       0  422M    0% /dev
tmpfs            99M    2,8M   96M    3% /run
/dev/mmcblk2p1  7,0G    1,7G  5,0G   25% /
tmpfs           493M    1,5M  491M    1% /dev/shm
tmpfs           5,0M       0  5,0M    0% /run/lock
tmpfs           493M    493M     0  100% /tmp
/dev/zram1       47M     29M   15M   66% /var/log
tmpfs            99M       0   99M    0% /run/user/1000
nanopi@nanopi:~$ cd /tmp/
nanopi@nanopi:/tmp$ ls -lha
insgesamt 8,0K
drwxrwxrwt 33 root   root    680 15. Okt 21:38 .
drwxr-xr-x 19 root   root   4,0K 27. Sep 22:17 ..
drwxrwxrwt  2 root   root     40  9. Okt 06:53 .font-unix
drwxrwxrwt  2 root   root     40  9. Okt 06:53 .ICE-unix
drwx------  3 root   root     60  9. Okt 06:53 systemd-private-c6eaf5160b5145dcb399122a353906ad-chrony.service-u701bf
drwx------  3 root   root     60  9. Okt 06:53 systemd-private-c6eaf5160b5145dcb399122a353906ad-systemd-logind.service-SdAIE3
-rw-r--r--  1 pihole pihole   14 12. Okt 12:15 tmp.0Ezn08dnAF
drwx------  2 root   root     80 15. Okt 12:23 tmp.9HQ4C4Tdyq
drwx------  2 root   root     80 12. Okt 20:29 tmp.Aa3TphoHDr
drwx------  2 root   root     80 10. Okt 20:03 tmp.AwBn239Nqk
drwx------  2 root   root     80 15. Okt 11:30 tmp.aXnfb9IJno
drwx------  2 root   root     80 15. Okt 20:39 tmp.CaUYmgHbQx
drwx------  2 root   root     80 14. Okt 20:13 tmp.CSgsE1C7U3
drwx------  2 root   root     80 10. Okt 20:13 tmp.CWJE3EQfal
drwx------  2 root   root     80 12. Okt 12:19 tmp.dwh3AGJeqR
drwx------  2 root   root     80 15. Okt 21:08 tmp.ES2wFMlGBg
drwx------  2 root   root     80 15. Okt 14:33 tmp.GlsdKu2nqI
drwx------  2 root   root     80 13. Okt 11:05 tmp.jL4elRk5kI
drwx------  2 root   root     80 15. Okt 20:40 tmp.MVzWRg1zBZ
drwx------  2 root   root     80 11. Okt 21:30 tmp.ofHmEZYfIS
drwx------  2 root   root     80 15. Okt 14:20 tmp.S1xaXwJDL7
drwx------  2 root   root     80 14. Okt 13:22 tmp.SghEPx1hF1
drwx------  2 root   root     80 12. Okt 22:58 tmp.sPz8ZUZl5v
drwx------  2 root   root     80 15. Okt 20:47 tmp.ttnwjtg5TR
drwx------  2 root   root     80 10. Okt 20:39 tmp.u4y5z1brBm
drwx------  2 root   root     80 12. Okt 22:47 tmp.uMXNkZScfo
drwx------  2 root   root     60 15. Okt 21:38 tmp.UYkFBo6MtC
drwx------  2 root   root     80 13. Okt 21:40 tmp.V28487if58
drwx------  2 root   root     60 15. Okt 21:38 tmp.W2LMfpcEje
drwx------  2 root   root     80 13. Okt 21:30 tmp.XR20h1l31R
drwx------  2 root   root     80  9. Okt 21:23 tmp.YWLAsQd56q
drwx------  2 root   root     80 15. Okt 21:33 tmp.zuoBWMSFWs
drwxrwxrwt  2 root   root     40  9. Okt 06:53 .X11-unix
drwxrwxrwt  2 root   root     40  9. Okt 06:53 .XIM-unix

This PR removes the temporary directory after FTL is installed

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)
6. I have checked that another pull request for this purpose does not exist.
7. I have considered, and confirmed that this submission will be valuable to others.
8. I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
9. I give this submission freely, and claim no ownership to its content.

---

• I have read the above and my PR is ready for review. Check this box to confirm