Add RHEL 8 support with multi-distro package management

[Copilot]

Adds support for RHEL 8, CentOS 8, Rocky Linux 8, and AlmaLinux 8. Action was previously hardcoded to Ubuntu's apt-get and libssl1.1.

Changes

• OS detection: Parse /etc/os-release to identify distribution
• Package manager selection: Use dnf/yum + compat-openssl11 for RHEL-based systems, apt-get + libssl1.1 for Debian-based
• Critical fix: Renamed VERSION → BUILDCACHE_VERSION to prevent conflict when sourcing /etc/os-release (which sets VERSION to OS version, breaking download URLs)
• OpenSSL detection: Replace ldconfig -p with explicit find across lib paths to avoid permission issues

Variable conflict example

Before fix, sourcing /etc/os-release would overwrite buildcache version:

VERSION="v0.31.5"           # buildcache version
. /etc/os-release           # sets VERSION="24.04.3 LTS (Noble Numbat)"
# Now VERSION contains OS version, causing malformed download URL

After fix:

BUILDCACHE_VERSION="v0.31.5"  # preserved across os-release sourcing
. /etc/os-release              # sets VERSION but doesn't affect us

Backward compatibility

Ubuntu/Debian workflows unchanged. Unsupported distros fall back to Debian behavior with warning.

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[magnesj]

Not able to use buildcache on RHEL8 due to GLIBC requirements