fix: Remove rust version pin for integration test job

[paleolimbot]

Apply apache/arrow#45790 / apache/arrow-rs#7292 to fix the failing integration test job.

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   > [integration 5/5] RUN ARCHERY_INTEGRATION_WITH_NANOARROW="0"     conda run --no-capture-output     /arrow-integration/ci/scripts/integration_arrow_build.sh     /arrow-integration     /build:
  4.312   Downloaded cfg-if v1.0.0
  4.314   Downloaded async-stream v0.3.6
  4.317   Downloaded anstyle v1.0.10
  4.319   Downloaded adler2 v2.0.0
  4.321   Downloaded http-body v1.0.1
  4.374 error: package `half v2.5.0` cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.77.2
  4.374 Either upgrade to rustc 1.81 or newer, or use
  4.374 cargo update half@2.5.0 --precise ver
  4.374 where `ver` is the latest version of `half` supporting rustc 1.77.2
  4.386 ERROR conda.cli.main_run:execute(125): `conda run /arrow-integration/ci/scripts/integration_arrow_build.sh /arrow-integration /build` failed. (See above for error)
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  failed to solve: process "/bin/bash -c -l ARCHERY_INTEGRATION_WITH_NANOARROW=\"0\"     conda run --no-capture-output     /arrow-integration/ci/scripts/integration_arrow_build.sh     /arrow-integration     /build" did not complete successfully: exit code: 101