fish: just run fastfetch, if found in path

fish: just run fastfetch, if found in path

We're using command -v as a tradeoff here, because speed matters:

❯  time test -f /usr/bin/fastfetch
________________________________________________________
Executed in    3.00 micros    fish           external
   usr time    3.00 micros    3.00 micros    0.00 micros
   sys time    2.00 micros    2.00 micros    0.00 micros

❯  time which fastfetch
which: no fastfetch in (/home/andy/Applications:/home/andy/go/bin:/home/andy/.cargo/bin:/home/andy/.local/bin:/home/andy/.spicetify:/home/andy/bin:/usr/local/bin:/usr/bin:/bin:/opt/android-sdk/platform-tools:/home/andy/.local/bin:/home/andy/Applications:/home/andy/go/bin:/home/andy/.cargo/bin:/opt/android-sdk/platform-tools:/home/andy/.local/bin)
________________________________________________________
Executed in  926.00 micros    fish           external
   usr time  751.00 micros    0.00 micros  751.00 micros
   sys time  202.00 micros  202.00 micros    0.00 micros

❯  time command -v fastfetch
________________________________________________________
Executed in   23.00 micros    fish           external
   usr time   17.00 micros   17.00 micros    0.00 micros
   sys time    8.00 micros    8.00 micros    0.00 micros