converter: fix plain HTTP in conversion

[bergwolf]

We should just use plain http when user sets the option.

Before fix:

INFO[2025-09-01T11:28:35Z] converted image localhost:5000/busybox:latest-rafs-v5 , elapse 198.159695ms  module=converter
INFO[2025-09-01T11:28:35Z] pushing image localhost:5000/busybox:latest-rafs-v5  module=converter
WARN[2025-09-01T11:28:35Z] Operation failed, will retry                  attempt=2 error="failed to do request: Head \"https://localhost:5000/v2/busybox/blobs/sha256:60fab8113b53ba4a8d9d26e4d3fe6b2af1fe06b5c5c704c7fe1748b4531a23cb\": http: server gave HTTP response to HTTPS client" retry_delay=5s total_attempts=3
WARN[2025-09-01T11:28:40Z] Operation failed, will retry                  attempt=3 error="failed to do request: Head \"https://localhost:5000/v2/busybox/blobs/sha256:60fab8113b53ba4a8d9d26e4d3fe6b2af1fe06b5c5c704c7fe1748b4531a23cb\": http: server gave HTTP response to HTTPS client" retry_delay=5s total_attempts=3
ERRO[2025-09-01T11:28:45Z] Operation failed after all attempts           error="failed to do request: Head \"https://localhost:5000/v2/busybox/blobs/sha256:37c94177fc5ccea45c08fa808b4007c949f3086124ef448792e58e1034d25802\": http: server gave HTTP response to HTTPS client" total_attempts=3
ERRO[2025-09-01T11:28:45Z] Push failed after all attempts                error="failed to do request: Head \"https://localhost:5000/v2/busybox/blobs/sha256:37c94177fc5ccea45c08fa808b4007c949f3086124ef448792e58e1034d25802\": http: server gave HTTP response to HTTPS client"
INFO[2025-09-01T11:28:45Z] try to push with plain HTTP for localhost:5000/busybox:latest-rafs-v5  module=converter
INFO[2025-09-01T11:28:45Z] pushed image localhost:5000/busybox:latest-rafs-v5, elapse 10.041605774s  module=converter

After fix:

INFO[2025-09-01T11:29:24Z] converted image localhost:5000/busybox:latest-rafs-v5 , elapse 189.642906ms  module=converter
INFO[2025-09-01T11:29:24Z] pushing image localhost:5000/busybox:latest-rafs-v5  module=converter
INFO[2025-09-01T11:29:24Z] pushed image localhost:5000/busybox:latest-rafs-v5, elapse 6.277371ms  module=converter

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.55%. Comparing base (4909a90) to head (0740165).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
contrib/nydusify/pkg/converter/converter.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1749      +/-   ##
==========================================
- Coverage   55.56%   55.55%   -0.01%     
==========================================
  Files         197      197              
  Lines       53108    53111       +3     
  Branches    44443    44443              
==========================================
- Hits        29508    29506       -2     
- Misses      22131    22135       +4     
- Partials     1469     1470       +1     

Files with missing lines	Coverage Δ
contrib/nydusify/pkg/converter/converter.go	64.46% <0.00%> (-0.48%)	⬇️

... and 2 files with indirect coverage changes

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[imeoer]

LGTM!

[Zephyrcf]

LGTM! I will check the failed ci.

[BraveY]

LGTM!