Relax project msrv; CI test on msrv, stable, and nightly

[pchickey]

• wstd's msrv is now backed up by 1 to 1.88, which is the aws sdk's current msrv, in preparation of upstreaming wstd-aws as part of the official distribution.
• Backing off to 1.88 required one minor change in test-programs to take a dep for a not-yet-stabilized locking method on files. Backing off to 1.87 would require minor syntactical changes in wstd itself, which of course is possible, but I'm not motivated to keep going backwards for things like that unless we have a use case that requires it
• the install rust action had a bug in the sed for parsing msrv, which is now fixed
• CI was testing a matrix of host OSs, but just stable rust. I don't feel strongly that we need to test on a matrix of host OSs since wasmtime is taking care of cross-OS compatibility, so I eliminated that dimension of the matrix, and I made the matrix test msrv, stable, and nightly rust. If nightly breaks we can pin it to an older nightly or eliminate that column.