Welcome,
You can use function dtostrf to convert float to string, then function strcat to add the "°C" behind, or you can simply use function sprintf:
float val = 12.3456789;
char str[8]; // buffer to hold string "-XX.X°C\0"
dtostrf( val, 5, 1, str );
strcat( str, "°C" );
// or
sprintf( str, "%.1f°C", val );
The downside of sprintf is, it uses a lot (1 to 2kB IIRC) of program space.
And to avoid buffer overflow if temperature sensor gives an incorrect reading like 12345°C (IDK if it's even possible), you must check if the temperature is in a range that can fit inside the buffer (so -99 to 999, less than 4 characters in front of the dot), or increase buffer size.