`cmd/enter`, `cmd/run`: use `/usr/bin/env <shell-name>` instead of `/bin/<shell-name>`
Fixes #1431. Also replaces runFallbackCommands in src/cmd/run.go with /usr/bin/env .
Build succeeded. https://softwarefactory-project.io/zuul/t/local/buildset/b89138694f784114b330ed1deaa27dea
:heavy_check_mark: unit-test SUCCESS in 6m 48s :heavy_check_mark: unit-test-migration-path-for-coreos-toolbox SUCCESS in 3m 14s :heavy_check_mark: unit-test-restricted SUCCESS in 5m 57s :heavy_check_mark: system-test-fedora-rawhide SUCCESS in 29m 38s :heavy_check_mark: system-test-fedora-39 SUCCESS in 31m 11s :heavy_check_mark: system-test-fedora-38 SUCCESS in 31m 44s
Build succeeded. https://softwarefactory-project.io/zuul/t/local/buildset/0099018b319546999e1616881d702e18
:heavy_check_mark: unit-test SUCCESS in 6m 31s :heavy_check_mark: unit-test-migration-path-for-coreos-toolbox SUCCESS in 3m 09s :heavy_check_mark: unit-test-restricted SUCCESS in 5m 51s :heavy_check_mark: system-test-fedora-rawhide SUCCESS in 31m 16s :heavy_check_mark: system-test-fedora-39 SUCCESS in 30m 24s :heavy_check_mark: system-test-fedora-38 SUCCESS in 30m 08s