build: bump up MSVR for const generics

[laysakura]

error[E0658]: const generics are unstable
  --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/postcard-0.7.3/src/accumulator.rs:65:34
   |
65 | pub struct CobsAccumulator<const N: usize> {
   |                                  ^
   |
   = note: see issue #74878 <https://github.com/rust-lang/rust/issues/74878> for more information

error[E0658]: const generics are unstable
  --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/postcard-0.7.3/src/accumulator.rs:93:12
   |
93 | impl<const N: usize> CobsAccumulator<N> {
   |            ^
   |
   = note: see issue #[74](https://github.com/laysakura/serde-encrypt/runs/5999148894?check_suite_focus=true#step:9:74)8[78](https://github.com/laysakura/serde-encrypt/runs/5999148894?check_suite_focus=true#step:9:78) <https://github.com/rust-lang/rust/issues/74878> for more information

   Compiling serde_cbor v0.11.2
error: aborting due to 2 previous errors