[INFRA]: Declaration commands mask return values in CI scripts (SC2155)

### Is this a duplicate?

- [x] I have searched the [open issues](https://github.com/NVIDIA/cccl/issues) and this is not a duplicate

### Overview

76 sites across 37 CI scripts combine variable declaration (`local`, `readonly`, or `export`) with command substitution on the same line. The exit code of the command substitution is masked by the declaration keyword (which always returns 0).

All affected scripts use `set -euo pipefail`. Combining declaration with substitution silently defeats that intent — if the command fails, the script continues with an empty or partial value instead of aborting.

### Details

**Pattern:**
```bash
# Current — if cd fails, $? is still 0 due to readonly:
readonly ci_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

# Fixed — command failure is visible to set -e:
ci_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly ci_dir
```

Same applies to `local` and `export`:
```bash
# local:
local start_time
start_time=$(date +%s)

# export:
CXX="$(which g++)"
export CXX
```

**Scope:**

| Pattern | Count | Files |
|---------|-------|-------|
| `readonly var="$(cmd)"` | ~55 | ci/util/artifacts/\*.sh, ci/util/workflow/\*.sh, ci/upload_\*.sh |
| `local var="$(cmd)"` | ~15 | ci/pretty_printing.sh, ci/util/memmon.sh, ci/pyenv_helper.sh |
| `export var="$(cmd)"` | ~3 | ci/build_cuda_cccl_wheel.sh |
| `readonly var=$(cat <