Embed epoch timestamp to SHA256SUMS

Lines 276 to 279 in 22cee8d

    
           epoch := st.ModTime().UTC() 
        
           if sourceDateEpoch != nil { 
        
           	epoch = *sourceDateEpoch 
        
           }

ModTime() is not reliable on git repo, so it should be embedded in the hash file.

e.g.,

0000000000000000000000000000000000000000000000000000000000000000  /repro-get?epoch=1639958400

	epoch := st.ModTime().UTC()
	if sourceDateEpoch != nil {
	epoch = *sourceDateEpoch
	}