only create & bind build_logs_dir and shared_fs_path if they are specified

[boegel]

This fixes early failures in the build job when shared_fs_path or build_logs_dir are not specified in the bot configuration.

Only trace of this in job output is the bot/build.sh script finishing very early (because set -e is used in bot/build.sh)

bot/build.sh: SHARED_FS_PATH=''
mkdir: missing operand
Try 'mkdir --help' for more information.
bot/build.sh finished

[eessi-bot]

Instance eessi-bot-citc-aws is configured to build:

• arch x86_64/generic for repo eessi-2021.12
• arch x86_64/generic for repo eessi.org-2023.06-compat
• arch x86_64/generic for repo eessi.io-2023.06-compat
• arch x86_64/generic for repo eessi-2023.06-software
• arch x86_64/intel/haswell for repo eessi-2021.12
• arch x86_64/intel/haswell for repo eessi.org-2023.06-compat
• arch x86_64/intel/haswell for repo eessi.io-2023.06-compat
• arch x86_64/intel/haswell for repo eessi-2023.06-software
• arch x86_64/intel/skylake_avx512 for repo eessi-2021.12
• arch x86_64/intel/skylake_avx512 for repo eessi.org-2023.06-compat
• arch x86_64/intel/skylake_avx512 for repo eessi.io-2023.06-compat
• arch x86_64/intel/skylake_avx512 for repo eessi-2023.06-software
• arch x86_64/amd/zen2 for repo eessi-2021.12
• arch x86_64/amd/zen2 for repo eessi.org-2023.06-compat
• arch x86_64/amd/zen2 for repo eessi.io-2023.06-compat
• arch x86_64/amd/zen2 for repo eessi-2023.06-software
• arch x86_64/amd/zen3 for repo eessi-2021.12
• arch x86_64/amd/zen3 for repo eessi.org-2023.06-compat
• arch x86_64/amd/zen3 for repo eessi.io-2023.06-compat
• arch x86_64/amd/zen3 for repo eessi-2023.06-software
• arch aarch64/generic for repo eessi-2021.12
• arch aarch64/generic for repo eessi.org-2023.06-compat
• arch aarch64/generic for repo eessi.io-2023.06-compat
• arch aarch64/generic for repo eessi-2023.06-software
• arch aarch64/neoverse_n1 for repo eessi-2021.12
• arch aarch64/neoverse_n1 for repo eessi.org-2023.06-compat
• arch aarch64/neoverse_n1 for repo eessi.io-2023.06-compat
• arch aarch64/neoverse_n1 for repo eessi-2023.06-software
• arch aarch64/neoverse_v1 for repo eessi-2021.12
• arch aarch64/neoverse_v1 for repo eessi.org-2023.06-compat
• arch aarch64/neoverse_v1 for repo eessi.io-2023.06-compat
• arch aarch64/neoverse_v1 for repo eessi-2023.06-software