pin rust toolchain to 1.47

[robinvd]

this is required as librespot panics on newer rust versions, see

• Panic: zero-initalisation of ov_callbacks #719
• Spotifyd crashes after update (to 0.3.0) #765
• https://bugs.archlinux.org/task/69308?project=5&string=spotifyd

can be reverted after librespot-org/librespot-tremor#1

[mainrs]

Make sure to subscribe to the issue to revert once resolved!

[robinvd]

Yes did that, maybe a good idea if someone else from @Spotifyd/maintainers also does that