bug(bom): overwrite epoch if srcEpoch is 0

[DmitriyLewen]

Description

There are cases when pkg.epoch is not 0, but pkg.srcEpoch in 0.
e.g.:

root@2ffdd2cb0e31:/# cat /var/lib/dpkg/status | grep bsdutils -A 18
Package: bsdutils
Essential: yes
Status: install ok installed
Priority: required
Section: utils
Installed-Size: 356
Maintainer: util-linux packagers <[email protected]>
Architecture: amd64
Multi-Arch: foreign
Source: util-linux (2.38.1-5)
Version: 1:2.38.1-5+b1
Pre-Depends: libc6 (>= 2.34), libsystemd0
Recommends: bsdextrautils
Description: basic utilities from 4.4BSD-Lite
 This package contains the bare minimum of BSD utilities needed for a Debian
 system: logger, renice, script, scriptlive, scriptreplay and wall. The
 remaining standard BSD utilities are provided by bsdextrautils.
Homepage: https://www.kernel.org/pub/linux/utils/util-linux/

In this case, in sbom mode we overwrite epoch(take epoch from pkg.epoch).

example:

➜ trivy -q image python:3.12.2 -f json --list-all-pkgs | jq '.Results[].Packages[] | select(.Name=="bsdutils")'
{
  "ID": "bsdutils@1:2.38.1-5+b1",
  "Name": "bsdutils",
  "Identifier": {
    "PURL": "pkg:deb/debian/[email protected]%2Bb1?arch=amd64&distro=debian-12.5&epoch=1",
    "UID": "fbbe7b0a3cbb79a3"
  },
  "Version": "2.38.1",
  "Release": "5+b1",
  "Epoch": 1,
  "Arch": "amd64",
  "SrcName": "util-linux",
  "SrcVersion": "2.38.1",
  "SrcRelease": "5",
  "Licenses": [
    ...
  ],
  "Maintainer": "util-linux packagers <[email protected]>",
  "Layer": {
    "DiffID": "sha256:0238a17903242951b944d01ab189a30ac237c715121f14565a2af8b078415d97"
  }
}
➜ trivy -q image python:3.12.2 -f cyclonedx -o report.cdx.json                                                 
➜ trivy -q sbom report.cdx.json -f json --list-all-pkgs | jq '.Results[].Packages[] | select(.Name=="bsdutils")'
{
  "ID": "bsdutils@1:2.38.1-5+b1",
  "Name": "bsdutils",
  "Identifier": {
    "PURL": "pkg:deb/debian/[email protected]%2Bb1?arch=amd64&distro=debian-12.5&epoch=1",
    "UID": "7f314baaeb4c1961",
    "BOMRef": "pkg:deb/debian/[email protected]%2Bb1?arch=amd64&distro=debian-12.5&epoch=1"
  },
  "Version": "2.38.1-5+b1",
  "Epoch": 1,
  "Arch": "amd64",
  "SrcName": "util-linux",
  "SrcVersion": "2.38.1",
  "SrcRelease": "5",
  "SrcEpoch": 1,
  "Licenses": [
    ...
  ],
  "Layer": {}
}