docker: update rust to 1.81 for distroless Dockerfile

[cinghioGithub]

When building the keylime_agent docker image, the process fails with the following error:

[...]
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 > [builder 12/14] RUN make RELEASE=1 TARGETDIR=target target/release/keylime_agent:
0.164 Package systemd was not found in the pkg-config search path. 0.164 Perhaps you should add the directory containing `systemd.pc' 0.164 to the PKG_CONFIG_PATH environment variable
0.164 No package 'systemd' found
0.164 cargo build --target-dir="target" --release
0.305 error: failed to parse lock file at: /src/rust-keylime/Cargo.lock 0.305
0.305 Caused by:
0.305   lock file version `4` was found, but this version of Cargo does not understand this lock file, perhaps Cargo needs to be updated?
0.305 make: *** [GNUmakefile:27: target/release/keylime_agent] Error 101
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Dockerfile.distroless:26
--------------------
  24 |     COPY . /src/rust-keylime/
  25 |     WORKDIR /src/rust-keylime
  26 | >>> RUN make RELEASE=1 TARGETDIR=target target/release/keylime_agent
  27 |
  28 |     # truly just for debugging purposes for the assembly stage
--------------------
ERROR: failed to build: failed to solve: process "/bin/sh -c make RELEASE=1 TARGETDIR=target target/release/keylime_agent" did not complete successfully: exit code: 2
Error response from daemon: No such image: keylime_agent:latest-distroless

Update the rust docker image to 1.81-bullseye.