Pin Rust toolchain version to 1.81.0

[FooIbar]

1.82.0 has a severe binary size regression (+36%)

    FILE SIZE        VM SIZE
 --------------  --------------
   +44%  +131Ki   +44%  +131Ki    .text
   +30% +10.7Ki   +30% +10.7Ki    .eh_frame
  +6.6% +2.82Ki  +6.6% +2.82Ki    .rodata
   +11% +2.58Ki   +11% +2.58Ki    .rela.dyn
   +16% +2.41Ki   +16% +2.41Ki    .data.rel.ro
  +190% +2.39Ki  +190% +2.39Ki    .gcc_except_table
   +22% +1.29Ki   +22% +1.29Ki    .eh_frame_hdr
   +16%    +176   +16%    +176    .rela.plt
   +12%    +168   +12%    +168    .dynsym
   +14%    +112   +14%    +112    .plt
   +14%     +56   +14%     +56    .got.plt
   +40%     +48   +40%     +48    .data
  +1.9%     +24  +1.9%     +24    .dynstr
   +14%     +16   +14%     +16    .gnu.version
  [ = ]       0  -0.3%      -8    .bss
  -3.6%     -16  -3.6%     -16    .dynamic
  [ = ]       0 -81.4% -2.36Ki    .relro_padding
   +36%  +154Ki   +35%  +152Ki    TOTAL

[revonateB0T]

Upstream issue link?

[FooIbar]

Upstream issue link?

rust-lang/rust#130320